/. Embryo!, exp. Morph. Vol. 62, pp. 259-275, 1981 259Printed in Great Britain © Company of Biologists Limited 1981

Postembryonic growth of the compoundeye of the cockroach

By MARK S. NOWEL1

From the Department of Zoology, University of Leicester

SUMMARY

The postembryonic growth of the compound eye of the cockroach Periplaneta americanainvolves increases in the size of the individual ommatidia as well as a 35-fold increase in thenumber of ommatidia. These ommatidia are added to the anterior, dorsal, and ventralmargins of the eye by means of an almost continuous process of cell division in the prolifera-tion zone in these margins. This proliferation phase is followed by a process of maturationof bundles of' pre-ommatidial' cells into mature ommatidia, a process which involves furthercell division. Processes involved in compound-eye development are investigated by eye margingrafting, histological techniques and cell proliferation studies.

INTRODUCTION

Following the emergence of a hemimetabolous insect from the egg, thecompound eye grows from stadium to stadium by a combination of increasesin ommatidial cell size and increases in total numbers of ommatidia (Boden-stein, 1953). Some insects such as Dixippus morosus add very few facets totheir growing eye (Friza, 1928) whereas others such as Sphodromantis bioculata(Yamanouti, 1933) and the dragonflies (Ando, 1957; Sherk, 1977, 1978 a, b)exhibit a spectacular increase in ommatidial numbers during their developmentfrom nymph to adult. The compound eye of a newly hatched Periplanetaamericana nymph contains just over 100 ommatidia, while the adult eye cancontain over 3500 (Fig. 1).

We have shown (Nowel & Shelton, 1980) that the eye margin of the cock-roach does not advance through adjacent head epidermis to recruit it into theexpanding compound eye. Rather, it acts as a 'budding zone' which generatescells to form new ommatidia. In the present study, the histology of the edge ofthe growing eye plus the levels of cell division in the eye and head epidermisare examined at various points of the moult cycle. The results of these observa-tions further substantiate the role of the eye margin as a budding zone.

Hyde (1972) has studied the appearance of the developing compound eyesof various stadia in the cockroach P. americana and has concluded that growth

1 Author's address: Department of Zoology, University of Leicester, Leicester LEI 7RH,U.K.

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Fig. 1. The right compound eye (ce) of a P. americana adult. The inset shows anewly hatched larva photographed at the same magnification. The larval eye containsjust over 100 ommatidia while that of the adult contains over 3500. Growth of thecompound eye occurs both by increase in size of the ommatidia and by additionof new ommatidia. Specimens were prepared by freeze drying to show individualfacets clearly, a, antenna; o, lateral ocellus. Bar represents 0-25 mm.

occurs all around the perimeter of each eye, using as her criterion the presenceof an unpigmented zone seen to surround the eyes of living animals (Fig. 2,3 c, d). Maturacion of ommatidia and their addition to pre-existing ones already

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present in the previous instar was said to occur in this zone of maturation whichsurrounds the eye. Anderson (1976) has since offered histological evidence tocontradict this: histological sections show that zones of growth and maturationare not present along the posterior margin of the eye, but are confined to theanterior and dorsal borders. The experiments to be described were designed todistinguish between the two alternatives. The results of these experiments are inagreement with Anderson's (1976) histological observations: the compoundeye of the cockroach grows along its dorsal, anterior, and ventral faces, butnot along its posterior edge.

MATERIALS AND METHODS

Maintenance of cockroach stocks

Cultures of P. americana were maintained under conditions of constanttemperature (24 °C) and an alternating cycle of 12 h light/12 h dark, and fed on adiet of rat pellets and water.

Surgical techniques

Newly moulted third-fifth instar nymphs were selected for operations,anaesthetized in small glass vials cooled on ice for 10-20 min, and immobilizedon a bed of plasticine. Excisions of integument grafts were made using a razorblade fragment (Gillette francais) supported in a pin vice, and transferred tohomotopic sites prepared in host animals by removing integument of equalsize and shape. The grafts, consisting of eye margin and adjacent head epidermis,were held in place using a small droplet of melted insect wax (Krogh & Weis-Fogh, 1951). Wild-type and lavender (Ross, Cochran & Smyth, 1964) stocks ofP. americana were used in graft exchanges.

Tissue preparation

Eye material was prepared by fixing in alcoholic Bouin, embedded in paraffin,sectioned at 10 /m\ and stained with Delafield's haematoxylin and eosin (Pantin,1969). Alternatively, material was fixed in a glutaraldehyde/paraformaldehydemixture (Karnovsky, 1965) buffered in a phosphate buffer (Hayat, 1970) atpH 7-4 for 2-4 h, and then postfixed in phosphate-buffered 1 % osmiumtetroxide for 2-3 h, following which it was dehydrated in an acetone series andembedded in Spurr's resin. Semithin (1 /tm) sections were cut using a HuxleyUltramicrotome with glass knives, and stained with 1 % toluidine blue in 1 %borax.

Colchicine studies

In order to investigate the temporal aspects of cell proliferation within thedeveloping eye during the moult cycle, and to locate areas where cell divisionoccurs, a 1 % colchicine solution (0-5 fi\ per 0-1 g live weight of animal) made up

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in insect saline (Hoyle, 1953) was injected into cockroach nymphs through adrawn micropipette inserted into an antenna. P. americana larvae of inter-mediate stages (fifth, sixth & seventh instar nymphs) were injected at differentpoints during their moult cycle (0, 1, 3, 5, 7, 9, 14, 21 and 28 days following theprevious moult). Nymphs at these intermediate stages have an intermoultperiod approximately one month in duration (Biellmann, 1960), and so con-sideration of the 28-day animals was restricted to those nymphs which wereabout to moult as determined by a cloudy appearance of the eyes (Flint &Patton, 1959). Following injection with colchicine, the cut antenna was sealedwith insect wax (Krogh & Weis-Fogh, 1951); 12 h later, the animals werefixed for wax histology (three-six animals for each of the nine stages). Paraffin-embedded specimens were cut in a plane perpendicular to the growing dorsaledge of the eye. For a quantitative investigation, every third 10/*m section wasexamined for mitotic figures.

Dividing cells were scored in each of four areas: the head epidermis adjacentto the eye margin (a zone of epidermis extending 375 /tm from the border of theeye - an arbitrary but convenient distance for examining microscopically); theproliferation zone of the eye margin (P.Z.); the maturing zone of the eye (M.Z.)the mature eye. The appearance and location (at the cuticular inner surface)of dividing cells is the same in all four regions examined. (Additional samples,some treated with colchicine and some untreated, from each stage were fixed andembedded in Spurr's resin and sectioned at 1 fim for light microscopy.)

Photography

Experimental animals were photographed on a Zeiss Tessovar Photo-macrographic Zoom system. Sectioned material was photographed on a ZeissPhotomicroscope II.

RESULTS

Location of the growth zone in the eyes of P. Americana

Of the 50 nymphs on which operations had been performed to exchangewild-type with lavender eye margins, 39 survived to the imago. Four of theseshowed no donor-pigmented ommatidia and were discarded. Thirty-five animalshad a patch of ommatidia with donor-specific pigment in their compound eyes.This graft-derived eye tissue was visible from the first post-operative moult.As the eye grew along its margins, donor-phenotype ommatidia were added bythe implanted segment of eye margin, while host ommatidia were added by thenative eye margin. A clear boundary between graft- and host-derived tissues,visible on the basis of pigment differences, is thus formed between eye tissues ofthese different origins. These boundary lines effectively demonstrate the direc-tions of eye expansion (Figs. 2, 3). In order to visualize this expansion, thegraft borders of these 35 mature chimeric eyes were projected onto a photo-

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graph of an adult eye, and traced onto the corresponding region of the photo-graph (Fig. 4).

In the dorsal half of the eyes examined in this series of experiments, growth ismost extensive in the anterodorsal region of the compound eye. Here there isthe greatest increase in the linear dimension of the margin resulting in thedivergence of graft/host border lines as they approach the margin (see Fig. 4),and the numbers of rows of new ommatidia added radially in each succeedingstadium is at its greatest in the anterodorsal quadrant. Relatively few rows ofnew ommatidia are added to the posterodorsal margin per moult (see Figs. 2, 3).

Of the 35 experimental animals, the chimeric compound eyes of 10 werephotographed following each post-operative moult. From analysis of the com-plete series of photographs, it is possible to define the extent of the growingmargin and the patterns of growth in different portions of the eye. Numbers ofommatidia added per moult in different parts of the eye were noted, and it waspossible to differentiate the growth resulting from addition of new ommatidiafrom that resulting from enlargement of pre-existing ones.

Because particular ommatidia are recognizable by their position with respectto the stable graft/host border, their position with respect to the changing eye/head-epidermis border can be followed. In succeeding stadia, particular facetsbecome further and further separated from the anterior, dorsal, anterodorsaland ventral eye margins by more and more newly added rows of facets. Thiswas noted in each of the ten chimerae produced. Ommatidial counts from thegraft/host border to these margins demonstrate addition of ommatidia duringthe post-operative instars. (Owing to the curvature of the ventrai portion of theeye, however, photographic presentation of these data is difficult.) No newommatidia are added to the posterior margin of the eye (Figs. 2, 3). These resultssupport the findings of Anderson (1976) against those of Hyde (1972).

Histology of the growing eye margin

Examination of sections through the edge of the eyes of P. americana nymphsshows retinal elements in various stages of developmental organization (Fig. 5).The proliferation zone is located at the extreme border of the eye along itsdorsal, anterior and ventral faces. Its cells are recognised by their undifferen-tiated and ungrouped appearance and by their dense packing. Mature omma-tidia have all their component cells in their correct proportions and positions,and they are found in the central and posterior part of the retina. The regionbetween the proliferation zone and the zone of mature ommatidia showspreommatidia in various stages of differentiation. This zone of maturingommatidia is operationally defined as a band of preommatidial bundles/ommatidia six bundles wide (approximately six rows of ommatidia are addedto the growing retina per moult during these intermediate larval instars:see Fig. 2).

Examining sections through eyes at various points during the moult cycle

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shows the development of a cuticular ridge under which the growing eye marginadvances. The epidermis adjacent to the eye margin (see Fig. 56-d) has theappearance of being displaced by the expanding eye and the extending ridge.The development of the ridge (the extent of which is best seen at ecdysis:Fig. 5 e) shows that the eye remains distinct from the epidermis even as the eyeexpands during the intermoult period.

Location and identification of mitotic figures

Typical sections through the eyes of colchicine-treated P. americana areshown in Fig. 6. Graphs showing the levels of mitotic activity in various regionsof the eye and head epidermis through the moult cycle following colchicinetreatment are shown in Fig. 7. Cells in the four regions of investigation (proli-feration zone, maturation zone, mature eye and head-capsule epidermis) whichare undergoing mitosis are recognizable in histological sections according tothe following criteria: (a) their basal ends are detached from the basementmembrane; (b) the cells are rounded up just beneath the cuticle, and (c) theyexhibit a basophilic condensation of chromosome material.

Cell division in the proliferation zone

The P.Z. in the eye margin is the only one of the four areas studied showingmitoses on all of the days examined (Fig. la). The level of mitotic activity inthis region reaches a maximum three days after the preceeding moult, risingsharply from day 0. This high level is maintained over a period of approxi-mately two weeks (analysis of the results showed that the levels of mitosis aresimilar on days 3, 5, 7, 9, and 14). Following this plateau phase, the levelgradually falls to a low point towards the end of the stadium.

At one point in the moult cycle the level of cell division within the P.Z.(and also in the M.Z., the mature retina, and the head epidermis) falls to zero.This point occurs shortly before moulting. The precise chronological point hasnot been defined because individual animals develop at slightly different rates1.

1 For footnote see p. 267.

FIGURE 2

A chimeric compound eye of P. americana generated by implanting a graft ofwild-type eye margin into theeye of a lavender nymph. The chimera was photographedat each post-operative stadium up to and including the adult (3 g). The series showsthe locations of the older and younger ommatidia and the numbers of rows ofommatidia added to the eye during each stadium. Note that more growth occurs inthe anterior than in the posterior eye margin. Note also the stability of the numbersof ommatidia along the posteroventral graft/host border (14 ommatidia at eachstadium) and the increase in ommatidia along the anteroventral border during thepost-opeiative stadia. This demonstrates that no new ommatidia are generated bythe posterior eye margin whereas the dorsal and anterior (as well as the ventral)eye margins produce new ommatidia during postembryonic development. A,anterior; D, dorsal; g, graft-derived ommatidia; h, host-derived ommatidia;o, lateral ocellus. Bar represents 0-25 mm.

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(a)

D

1 D

In 28-day animals which are about to moult as shown by a thick (35-40 /im)newly deposited cuticle underlying the old cuticle which is about to be sloughedoff, cell divisions are seen in the P.Z. The same age animals with a slightlyslower rate of development (as shown by a thinner (5-10 /im) newly depositedcuticle) show no cell divisions in structures derived from head ectoderm. It isclear that cell division ceases at the start of new cuticle deposition. Shortlythereafter, and even before ecdysis has occurred, cell division in the prolifera-tion zone resumes.

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Cell division in the maturing zone, the area of mature ommatidia, and the headepidermis

In these three regions, 80 % of the mitotic figures were found during a limitedpart of the moult cycle, namely at 9 days and 14 days after the previous moultor during the second quarter of the intermoult period (Fig. 1b, c). However,one 21-day animal showed an abnormal level of cell division compared withthe three others examined in that group which showed virtually no mitoses.Considering the variability of the actual length of the moult cycle1, such a21-day animal could be one developing at a slower rate which is still in the firsthalf of its intermoult period. If this individual were to be eliminated from con-sideration, then the mitoses occurring at days 9 and 14 into the (presumably28-day) intermoult period would comprise 97 % of the total mitoses in theseregions for the complete moult cycle.

DISCUSSION

The dimensions of the compound eye increase from instar to instar byaddition of new ommatidia along its dorsal, anterior, and ventral borders.Particular (marked) ommatidia become further and further removed from these

1 Several studies (Klein, 1933; Nigam, 1933; Gould & Deay, 1938; Gier, 1947; Willis,Riser & Roth, 1958; Biellmann, 1960) have demonstrated the variability of the intermoultperiod. In the present and in other work conducted concurrently (Bray, 1978), some indivi-duals within a group of cockroaches moult much later than the majority. There is no knownway of precise staging in P. americana. The range of variability in the levels of cell divisionshown on particular days of the moult cycle may in part be due to differing rates ofdevelopment of the animals examined at these stages.

FIGURE 3

Chimeric compound eyes of P. americana showing the growth at the eye margins.By comparing the numbers of ommatidia between a fixed position (the graft/hostborder) and the eye margin in the adults (b, d) with those in young larvae (a, c),it is apparent that new ommatidia have been added at the anterior, dorsal andanterodorsal eye margin. Counts of numbers of facets between a particular omma-tidium (arrows, Figs. 3 c, d) and the posterior margin of the eye always remainedconstant from moult to moult. No new ommatidia are produced at the posteriormargin. Note the band of unpigmented tissue surrounding the eye, which Hyde(1972) called the 'growing zone'. Note also the asymmetry of eye growth: moregrowth occurs in the anterior than in the posterior regions of the eye, as determinedby the longer graft/host border at the anterior edge of the graft.

(a) Larva after a single post-operative moult.(b) Same animal photographed in (a) but after six post-operative moults to the

adult.(c) Larva after two post-operative moults.{d) Same animal photographed in (c) but after six post-operative moults to the

adult.A, anterior; D, dorsal; g, graft-derived ommatidia; h, host-derived ommaditia.Bar represents 0-25 mm.

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Fig. 4. Photograph (a) and drawing (6) show the dorsal half of the right compoundeye of P. americana. Outlines of graft/host borders of a number of adult chimerasgenerated by homotopic grafting between wild-type and lavender nymphs havebeen traced onto these pictures to demonstrate the direction of postembryonicgrowth of the compound eye. The growth of this half of the eye is predominantlyin an anterodorsal direction. A, anterior; D, dorsal. Bar represents 0-5 mm.

FIGURE 5

Micrographs showing semithin sections through the left eye and head epidermis ofP. americana nymphs fixed at different times during the intermoult period showthe changing eye/epidermis interface.

(a) Newly moulted. The proliferation zone fPZ) and maturation zone (MZ) arevisible in the eye. The compound eye is not clearly separated from the epidermis (E)at this stage, c, cuticle.

(b) 7 days post ecdysis. The basement membrane (bm) of the eye is visible betweenthe eye margin and a fold of epidermis adjacent to it. The eye tissue extends to asmall cuticular ridge (arrow) which marks the boundary between it and the epider-mis, c, cuticle; M.Z., maturation zone; P.Z. proliferation zone.

(c) 28 days post ecdysis. The cuticular ridge (arrow) is extended and the prolifera-tion zone (P.Z.) underlies it. Note the epidermal bristle (b) penetrating the cuticle(c) at the eye margin. Its appearance indicates that the head epidermis is beingpushed ahead rather than being recruited by the expanding eye. M.Z. maturation zone.

(d) Pre-apolysis. Note the continued clear division between the eye and headepidermis (E). The folded appearance of the epidermis lying ahead of the cuticularridge (arrow) suggests it is being pushed back rather than being recruited by thegrowing eye. c, cuticle; M.Z. maturation zone; P.Z. proliferation zone.

(e) At apolysis, which marks the beginning of the moulting process, new cuticle(we) deposition has just begun. Note the extent of the cuticular ridge (arrow) of theold cuticle (oc). E. epidermis; M.Z., maturation zone; P.Z. proliferation zone.

(/) Later as the moult is approached, the inner (new) cuticle (we) is thickened.The old, shedding cuticle has been lost in tissue preparation. E, epidermis; M.Z.maturation zone; P.Z., proliferation zone.Bars represent 10/an.

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Cockroach compound-eye development 271

margins of the eye while having the same number of facets separating themfrom the posterior margin. Growth in this direction is due to increase in omma-tidial size only.

Hyde's (1972) conclusion that the zones of growth and maturation completelysurround the retina must therefore be incorrect. The zone of growth lies onlyalong the dorsal, anterior and ventral borders of the compound eye (Anderson,1978).

A significant observation from these studies is that different regions of thegrowth zone produce ommatidia at different rates. There is more growth, forexample, along the anterodorsal edge of the eye than along the posterodorsaledge (see Figs. 2, 3; for instance, the specimen illustrated in Fig. 2 showedthat, during the post-operative instars to the adult, 49 rows of ommatidia wereadded along the graft/host border to the anterodorsal edge of the eye, whileonly 29 rows were added to the posterodorsal edge). This suggests that regionsalong the eye margin have some inherent properties or are under some regionalcontrol to govern the rate of production of new omrnatidia and therefore therate of expansion of the eye.

The pattern of mitoses in the P.Z. of the eye margin is different from that ofthe M.Z., the mature ommatidia, and the head epidermis (compare Fig. la-c).During the entire intermoult period, the cells of the P.Z. undergo only a briefcessation of division, immediately before moulting. In contrast, the cells of theother three regions considered, more closely resemble the pattern found in awide variety of other ectodermal epithelia {Rhodnius prolixus abdominalepithelium: Locke, 1964; Schistocercagregaria abdominal epithelium: Anderson,1976, 1978; Leucophaea maderae abdominal epithelium: Bray 1978). Here celldivision is confined to a discrete period within the moult cycle. Followingecdysis (or in the case of R. proxilus, the blood meal) there is no cell division forthe first one fifth to one third of the intermoult period. After this interval,mitotic activity begins and continues until the intermoult period is approxi-mately half over. At this point, the level of mitosis falls to zero where it staysfor the remainder of the moult cycle.

FIGURE 6

(a) Micrograph of a semithin horizontal section through the eye margin of a wildtype P. americana nymph to show the location of dividing cells (arrows). Nine daysafter moulting, the animal was treated with colchicine to arrest cell division, andfixed 12 h later. Mitotic figures are seen in the proliferation zone (P.Z.) and in thematuration zone (M.Z.). Note the cuticular ridge (cr) separating the head epidermisfrom the eye. (b) Micrograph of a 10 /*m thick section through the right eye andhead epidermis of a P. americana nymph treated with colchicine in the middle ofthe intermoult period (14 days post ecdysis). This period falls within the plateauperiod of high mitotic activity for the proliferation zone (P.Z.) and in the peakperiods of mitotic activity for the zone of maturing ommatidia (M.Z.), the matureretina (M.R.) and the head capsule epidermis (E). Mitotic figures (arrows) arevisible in all these areas. Note also the elongated cuticular ridge (cr) along whichthe preommatidia develop. It separates the retina from the head epidermis.Bars represent 10 /im.

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1-

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Fig. 7. Bar graphs to show the mitotic activity in (A) the proliferation zone and(B) the maturation zone of the eye margin, and (C) the head epidermis adjacent tothe retina of P. americana nymph. Numbers of mitotic figures per examined sectionwere plotted against the number of days after the previous moult on which theanimals were injected with colchicine. Each bar represents the average numberof mitotic figures/section derived from the examination of at least 100 sectionsfrom at least four different animals.

(A) In the proliferation zone, dividing cells are observed on all examined days ofthe moult cycle. From a low point just before ecdysis, numbers of divisions steeplyrise to a plateau from days 3-14, and then gradually fall again.

(B) Dividing cells in the region of maturing ommatidia appear during a restrictedportion of the moult cycle only. Cell divisions begin when one quarter of theintermoult period has passed, and end when the animal is about to moult. Thispattern is similar to that qualitatively observed (but not quantified.for technicalreasons) in the mature retina.

(C) Dividing cells in the head epidermis are seen during a similarly restrictedportion of the moult cycle. The pattern of cell divisions in the head epidermis (as wellas in the regions of maturing and mature ommatidia) is therefore different from thatin the proliferation zone of the eye margin.

The results for the head epidermis of P. americana described here are com-parable to those described above, and mitoses in the M.Z. and mature omma-tidia follows a similar pattern. It seems reasonable to assume that cell divisionsin these three regions are under the same (possibly hormonal) control, thoughthe precise nature of these controls remains obscure.

The fundamentally different pattern of mitotic activity in the P.Z. (whichextends around the anterior, dorsal, and ventral borders of the eye) providesfurther evidence that it is a specialized source of new cells which are requiredthroughout the moult cycle for eye growth, i.e. a 'budding zone' for the produc-tion of cells to form new ommatidia in the expanding eye (Bodenstein, 1953).

Cockroach compound-eye development 273

During its postembryonic development, a continuous sequence of eventsoccurs in the growing zone of the retina to transform undifferentiated cells intomature ommatidia. In the locust S. gregaria, five stages of development can beidentified (Eley & Shelton, 1976): (a) Ungrouped cells. These are seen as aband of closely packed, undifferentiated cells immediately adjacent to the eye/head epidermis border, (b) Early cell clusters. Cells are grouped into rows ofbundles, or 'pre-ommatidia' (Imberski, 1967). (c) Late cell clusters. Cytologicaldifferentiation has made component cells in the pre-ommatidia distinguishable.(d) The developing rhabdom stage. Retinula cells form a rosette with microvillifrom each cell emanating centrally to form the rhabdom. (e) The matureommatidium. This consists of four distal cone cells with narrow cone-cellprocesses, two primary pigment cells and a variable number (7-13) of secondarypigment cells, and eight retinula cells surrounding a mature rhabdom (Eley &Shelton, 1976; Eley, 1978).

The formation of this final pattern in S. gregaria involves a programme ofmitosis in the eye similar to that found in the present studies on P. americana.That is, the proliferation zone is characterized by a high level of mitotic activitythroughout the intermoult period. Virtually all mitotic activity in the zone ofommatidial maturation occurs during the time mitoses in the P.Z. are at theirpeak, as do the mitoses in the mature retina (Anderson, 1978).

In Drosophila, the undifferentiated cells of the eye imaginal disc are trans-formed into ommatidia following two waves of mitoses. The first mitotic waveresults in "pre-clusters" of cells (retinula cells, 3, 3, 4, 5, 8) becoming post-mitotic. The second wave of mitoses results in the formation of the remainingcells required to complete the ommatidium, i.e., cone cells, pigment cells,bristle cells plus retinula cells 1, 6, 7 (Ready, Hanson & Benzer, 1976).

In P. americana there appears to be a single wave of mitoses (in the P.Z.)followed by a pulse of mitotic activity (in the M.Z.) which occurs once duringeach moult cycle. In contrast to the travelling waves, of mitoses in Drosophila,which are described as passing across the eye disc (Ready et ah 1976), the waveof cell division in the cockroach is a standing wave, remaining at the marginof the eye and generating new cells centrally to form the pre-ommatidia. It isunknown which ommatidial components are generated by the primary wave ofmitoses and which are generated by the secondary pulse. Anderson (1976)suggests that in S. gregaria, cells produced by the primary wave are retinulacells, (on the grounds that growing retinula axons are seen in the optic lobethroughout the moult cycle) and the cone cells (which are identifiable at theearliest stages). Cells produced by the pulse in the maturation zone are likelyto be any additional retinula cells, hair cells, or precursors of pigment cells.Cells dividing among the mature ommatidia are probably secondary pigmentcells, as other components are fixed in number by this stage.

As shown in Fig. 7, there is a low level of cell division in the head epidermisadjacent to the eye margin. The pattern of mitotic activity is similar to that in the

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mature retina and among the maturing ommatidia (compare Fig. 7 b, c) andsimilar to patterns of mitoses in other epidermal systems (see above).

These observations lend no support to the recruitment hypothesis (Hyde,1972). This hypothesis suggests that head-capsule epidermis undergoes a changein determination brought about by the advancing eye margin, as a result ofwhich epidermal cells produce ommatidia. Evidence against recruitment hasbeen presented (Nowel & Shelton, 1980) which shows that the growing eyemargin never approaches bristles close to the eye margin during a successionof larval instars. If there were an increased level of cell division between theeye margin and the bristles (which may be as close to the eye margin as a singlecell away) and /or a decreased level of cell death in this region (see Nowel, 1979which shows a very low level of cell death in the epidermis during the moultcycle), a stable distance between eye margin and head epidermis could be pre-served despite the occurrence of epidermal cell recruitment. The present obser-vations on cell division in the eye and head epidermis (Nowel, 1979) lend nosupport to this hypothesis.

My sincere thanks go to Dr Peter M. J. Shelton for all his help, advice and encouragementwhile working in his lab; to Beverely Hughes for help in preparing this manuscript and forexpert technical assistance; to Dr Ross for supplying our culture of lavender P. americana.We are grateful to the Science Research Council for its grant to P. M.J. Shelton.

REFERENCES

ANDERSON, H. (1976). Postembryonic development of the insect visual system. Ph.D. thesis,University of Leicester.

ANDERSON, H. (1978). Postembryonic development of the visual system of the locust, Schis-tocercagregaria.l. Pattern of growth and developmental interactions in the retina and opticlobe. / . Embryol. exp. Morph. 45, 55-83.

ANDO, H. (1957). A comparative study on the development of ommatidia in Odonata.Sci. Rep. Tokyo Kyoiku Daig., (B) 8, 174-216.

BIELMANN, G. (1960). £tude du cycles des mues chez Periplaneta americana. Bull. Soc. zool.Fr. 84, 340-351.

BODENSTEIN, D. (1953). Postembryonic development. In Insect Physiology (ed. K. D. Roeder),pp. 822-865. New York: Wiley.

BRAY, I. S. (1978). The epidermal cell cycle of Leucophaea maderae (Blattaria). Third yearproject, University of Leicester (unpublished).

ELEY, S. (1978). Postembryonic development of the insect retina-a light and electron micro-scope study. M. Phil, thesis, University of Leicester.

ELEY, S. & SHELTON, P. M. J. (1976). Cell junctions in the developing compound eye of thedesert locust Schistocerca gregaria. J. Embryol. exp. Morph. 36, 409-423.

FLINT, R. A. & PATTON, R. C. (1959). Relation of eye color to molting in Periplaneta ameri-cana L. Bull. Brooklyn ent. Soc. 54, 140.

FRIZA, F. (1928). Zur Frage der Farbung und Zeichnung des facettierten Insektenauges. Z.vergl. Physiol. 8, 289-336.

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(Received 23 June 1980, revised 7 November 1980)