J. Zool., Lond. (1969) 159,405-41 1

Prey and predators of the Wolf spider Lycosa Zugubris

WALTER D. EDGAR

Department of Zoology, University of Glasgow, Scotland

(Accepted 10 June 1969)

85 % of the prey ofLycosu lugubris (Walckenaer) consists of Diptera, Hemiptera and Araneae. The food taken appears to differ at different times of year; this is more probably due to changes in abundance of prey than to changes in preference by the spider. L. lugubris is not an active predator. It does not run down its prey but remains motionless and captures what comes within reach. It probably feeds infrequently in the field and it carries its prey for between one and two hours. An attempt was made to assess possible predators, and the species may be its own most important predator.

Contents

Introduction . . . . . . . . . . . . . . Prey of L. lugubris . . . . . . . . . . . .

Seasonal changes in prey of L. Iugubris Method of prey capture . . . . . . . . Duration and frequency of feeding

Predators of L. lugubris . . . . . . . . . . Summary . . . . . . . . . . . . . . References . . . . . . . . . . . . . .

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Introduction

Page . . . . . . . . 405 . . . . . . . . 405 . . . . . . . . 406 . . . . . . . . 407 . . . . . . . . 407 . . . . . . . . 408 . . . . . . . . 410 . . . . . . . . 411

Little information is available on the food taken by spiders in the field (Turnbull, 960; Kajak, 1965) or on the frequency with which these animals feed under natural conditions (Kajak, 1967). Likewise there is a dearth of detailed information about the predators of spiders.

In this paper an account is given of the prey of the Wolf spider Lycosa lugubris (Walcke- naer), the method of prey capture and the frequency of feeding. An attempt is made to assess the importance of possible predators of L. lugubris.

The work formed part of a four year population study on L. lugubris carried out in the oak woodland of the Ross Peninsula on the east bank of Loch Lomond in Stirlingshire, Scotland.

Prey of L. lugubris

During the course of the population study on L. lugubris a number of specimens were collected with food in their chelicerae. These prey organisms were removed and identified. The results are summarized in Table I. They indicate that most of the prey organisms come from relatively few taxonomic groups (cf. Turnbull, 1960) ; Diptera, Hemiptera and Araneae collectively constituting 85 % of the prey organisms.

31 405

406 W A L T E R D. E D G A R

TABLE I Prey of Lycosa lugubris

~~

Category Number Percentage

Diptera 39 32.8

"} 27.8 Hemiptera Miridae Auchenorhyncha

Anthocoris nemorum 3

Lycosa lugubris 191 24-4 Other spiders 10J Lepidoptera (adults) Lepidoptera (larvae) a \

Others Phalangida 1 15.1 Hymenop tera Collembola Plecoptera

Total 119

Seasonal changes in the prey of L. kgubris Turnbull (1960) found that the prey of the spider Linyphia triangularis differed at

different times of the year. In order to test whether this was true for Lycosa Iugubris it was necessary to compare organisms taken by one size category of spider, as it might be expected that different size categories would take different sizes of prey organism, thus

TABLE I1 Prey of adult females of Lycosa lugubris

Prey Apr. May Jun. Jul. Aug. Sep. Totals

0 1 0 0 7 Small Diptera L. lugubris 0 I21 1cl 1 0 2 13

Other Hemiptera 2 0 0 1 2 0 5 Other spiders 1 0 0 2 2 1 6 Large Diptera 1 2 1 P I \?I 2 14

Totals I 1 24 7 11 1 1 5 69

- 2 IT1 -

Auchenorhyncha 1 IEI 2 0 2 0 13

Others 2 El 0 2 1 0 11

* Numbers constituting more than 5% of total prey organisms.

invalidating comparison. Adult females are the only size category present from spring to autumn and the prey organisms taken by adult females are indicated in Table 11. The total number of prey organisms is relatively small but suggests that small Diptera, Aucheno- rhyncha and specimens of L. lugubris are taken in late spring and early summer, while

P R E Y A N D P R E D A T O R S O F L . L U G U B R I S 407

large Diptera are taken in late summer. The low numbers of prey organisms in June and September are to be expected as it is known from the life-cycle of L. lugubris (Edgar, in prep.) that most adult females are carrying egg sacs at these times and are taking little food.

It is not considered that the seasonal differences in the prey of L. lugubris represent a change in food preference of the spider but, as Turnbull (1960) found for Linyphia tr~angular~s, simply reflects the changes in abundance of prey organisms.

Method of prey capture

It is frequently stated, or implied, that wolf spiders are active predators which run down their prey (Savory, 1928 ; Nielsen, 1931 ; Comstock, 1948 ; Bristowe, 1958), because they can run quickly and have relatively good eyesight (Homann, 1931). However, Diptera and saltatory homopterans constitute a considerable proportion of the prey of L. lugubris, so it is unlikely that the species actively chases and catches its prey. Laboratory observa- tions have confirmed that the spider waits motionless and seizes the prey with one rapid motion when it comes within reach. Individuals react much more immediately to prey dropped in front of them than to an organism which walks slowly into their field of vision. This might be expected of an animal which captures Diptera.

The prey organism is seized by the front pair of legs and transferred to the chelicerae. If the spider fails to make the capture at the first attempt it waits, motionless, until the prey organism moves again, and then attempts to capture it.

These findings are in agreement with the conclusion reached by Cragg (1961) that spiders of the family Lycosidae were “behaving rather like Opiliones which Phillipson (1960) has shown largely depend on prey organisms making contact with them”. Cragg is, however, perhaps overstating the case when he says “the Lycosidae are highly sedentary organisms”.

Duration and frequency of feeding

Of the several thousand spiders collected from the field during the four years of the study few had food in their chelicerae. This observation suggests that L. 1ugubri.s feeds at infrequent intervals. To test this hypothesis three collections were made with the specific

TABLE I11

Numbers of individuals feeding in samples of Lycosa Iugubris

Category of L. lugubris Date No. collected No. feeding

Adult females with egg sacs 7.7.65 77 0 Adult females without egg sacs 26.4.66 25 2 2nd, 3rd and 4th instars 12.9.66 60 1

purpose of determining the proportion of spiders feeding (Table 111). Laboratory experi- ments were also carried out, to determine how long different stages of L. lugubris spent eating selected prey organisms; namely Drosophila and chironomids (Table IV). Using

408 W A L T E R D . E D G A R

this information, and making certain assumptions, an estimate can be made of the number of individuals one would expect to be feeding in a field sample. If the time available for feeding in the field is taken as ten hour/day, and the duration of carrying the prey organism for an adult female as one hour (Table IV), then in a sample of 25 adult female spiders, assuming that the spiders exhibit no special feeding preference within the ten-hour period, the mean number of spiders (x) feeding at any given time, assuming one prey organism is eaten per day, is given by:

25 10

= - = 2.5.

In the field sample two out of 25 adult females were feeding (Table HI), hence the assump- tion that one prey organism is eaten per day would appear to be valid. Laboratory feeding experiments (Edgar, in prep.) have indicated, however, that adult females growing at the same rate as in the field are in fact consuming the equivalent of three Drosophila per day.

TABLE IV

Duration of feeding of different stages of Lycosa Iugubris

Time when 4 No. of have finished

Category spiders tested Prey feeding (min)

Adult 9 12 Drosophila 58 Subadult 8 and 48 Drosophila 90 3rd instar 14 Chironomids 128

The discrepancy may be explained by the fact that the assumption that spiders are just as likely to be feeding at any time in the ten-hour period may not be biologically valid. This could be tested by comparing the numbers of spiders feeding in samples taken at different times of day.

Thus, in the light of the relatively long time during which the prey organism is carried, and the few spiders feeding in the field samples, it can be concluded that L. lugubris feeds relatively infrequently. This conclusion is in agreement with the statement by Savory (1964) that “. . . many a spider is overfed if given a fly daily”.

Predators of L. lugubris

L. lugubris overwinters twice before reaching maturity (Edgar, in prep.). Thus the overwintering spiders comprise of two distinct year groups. Subadult animals (the instar before the adult stage) of the one year group will readily eat the 2nd, 3rd and 4th instars of the younger year group and 16 % of the prey organisms taken by L. lugubris consisted of smaller members of their own species (Table I). Hence L. lugubris may be the most important predator of its own species; a finding which is in accord with Bristowe’s (1941) view, that the most important predators of spiders are other spiders.

The two year groups overwinter in the same area, among accumulations of leaf litter under oak trees, and it thus seemed possible that the population of small instars might

P R E Y A N D P R E D A T O R S O F L . L U G U B R I S 409

be decimated by predation of the larger individuals. Small instars of L. lugubris had a marked propensity to wedge themselves into crevices. This observation, coupled with the fact that the leaf litter represented a three-dimensional medium, suggested that the small instars might run in the “tunnels” within the litter while the larger instars run on the surface of the litter and consequently that the two age groups tend to be spatially separated by the three-dimensional medium of the leaf litter.

To test this hypothesis the following experiment was carried out: Two plastic boxes 15 x 8 x 6 cm were used. In one was placed a flat strip of expanded polystyrene. This was tight-fitting so that there were no gaps between the polystyrene and the sides of the box into which the small instars could wedge themselves. In the other box, flakes of expanded polystyrene were placed to simulate the three-dimensional medium of leaf litter. A supply of water was made available in both boxes, it being made available in the “leaves” both at the surface level and beneath the surface. Six 4th instar spiders were placed into each box. After they had settled down, a previously starved adult female was introduced into each box. The rate at which the small spiders were eaten in each box was noted. The experi- ment was carried out three times, once using subadult females as predators and twice using adult females. The experiment with subadult females was run for 23 hours and those with adult females for 20 and 52 hours respectively. With the subadult females as predators only one small spider was alive on the polystyrene “strip” at the end of the experiment while all six were still alive in the “leaves” after 23 hours. With the adult females as predators no small spiders were alive on the polystyrene “strip” at the end of the experi- ment while all 12 in the “leaves” were still alive. This suggests that a three-dimensional medium provides a substrate in which large and small instars are spatially separated and thus cannibalism is reduced. Young specimens of the Wolf spider Trochosu terricolu occur more frequently among leaf litter than on the surface of leaf litter where the adult animals occur (Breymeyer, 1961).

Among vertebrates, the most likely predators of L. lugubris are frogs, toads, shrews and birds.

Nine frogs and one toad were taken from the sampling areas and their gut contents examined. A large variety of arthropods was present in the gut contents. The toad had a subadult specimen of L. lugubris in its gut but no spiders were taken from the frogs. Avery (1966) examined the gut contents of thirty specimens of the Common lizard Lucerta vivipara. He found that spiders constituted the principle food. He compared the proportion of the various groups of invertebrates in the stomachs with those in the habitat in which the lizards lived. A fairly close correspondence was obtained between the two lists, the most mportant difference being that spiders of the genus Lycosa were abundant in the areas yet were not found in the stomachs. He suggested that Lycosu species are too fast-moving to be caught often in the field, but observed that they are, however, readily taken by lizards in captivity. This latter point illustrates the danger of making assumptions about feeding in the field from laboratory experiments.

Hence it would seem that amphibians and reptiles are probably of minor importance as predators of L. lugubris.

Shrews are fairly abundant in the sampling areas. Rudge (1968) in a recent paper on the food of Sorex uruneus states “. . . Araneae were never prominent” (in the gut contents). The manner in which shrews feed (Crowcroft, 1957; Rudge, 1968) makes it unlikely that a spider with the visual acuity of L. lugubris would frequently fall prey to them.

410 W A L T E R D. E D G A R

The importance of birds as predators of L. lugubris is difficult to assess. Most records concerning feeding in birds indicate that “spiders” were, or were not, eaten. Betts (1955) studied the food of titmice in oak woodlands. In May she found that Great tits (Parus major L.) were feeding mainly on the ground where lycosids with egg sacs were very abundant. Relatively few lycosids were found in the gizzard contents considering the abundance of the spiders. Great tits occur on the Ross Peninsula and if they selectively eat females with egg sacs they could be important predators of L. lugubris.

Adult and subadult specimens of L. lugubris are among the larger members of the terrestrial invertebrate fauna. They are well equipped for attack or defence and will retreat rapidly not only from animals larger than themselves, but also those which are slightly smaller. Adult specimens of the Wolf spider Trochosa ruricola have few invertebrate predators (Hackman, 1957). Invertebrate predators on the Ross Peninsula which are larger than L. lugubris, such as carabid beetles, gnaphosid and Trochosa spiders, are rather slow- moving compared with L. lugubris. Further, Breymeyer (1966a,b) has suggested that L. lugubris is temporally separated from the larger ground-dwelling spiders as the latter are mainly nocturnal.

In the autumn of 1965 a number of adult females, with and without egg sacs, were found to have white fungus-like growths on their cephalothoraxes and abdomens. These animals appeared to be in very poor condition. They drank large quantities of water and died within an hour or two if kept without water. In this respect they resembled the adult males in late June just prior to their disappearance. The white growth was identified as a yeast- like fungus ( S . A. Hutchinson, pers. comm.). Bristowe (1958) recorded the presence of a white fungus on certain species of spider and considered that the fungus, in fact, killed the animals. With regard to L. lugubris, healthy spiders, kept for several weeks with a number of dead spiders with fungus, were not attacked by the fungus. It would seem, therefore, that while the fungus may lead to a slightly premature death of L. lugubris the fact that it has only been found on adults, which would have died within a few weeks in any case, suggests that it will only attack spiders which are in a moribund condition and is not the prime cause of death.

Summary

85% of the prey of L. lugubris consists of Diptera, Hemiptera and Araneae. The food taken appears to differ at different times of the year. This probably reflects changes in abundance of prey organisms and not changes in preference by the spider. It is suggested that L. lugubris is not an active predator in that it does not run down its prey. Prey is captured in one sudden movement when it comes within reach of the motionless spider. The duration of carrying prey organisms varies with the size category of L. lugubris and the size of the prey but the mean time is probably between one and two hours. L. lugubris probably feeds infrequently in the field.

The predators of L. lugubris are considered. L. lugubris may be the most important predator of its own species but the three-dimensional medium of the leaf litter tends to separate spatially the two overwintering year groups and thus reduces cannibalism. Other invertebrates, mammals, reptiles and amphibians are probably of minor importance but birds may be important predators at certain times of the year. A fungal growth which occurs on adult males and females just before they die is probably a manifestation of the poor condition of the animals rather than the prime cause of death.

P R E Y A N D P R E D A T O R S O F L . L U G U B R I S 41 1

I should like to thank Dr A. F. G. Dixon and Dr S. A. Hutchinson for their helpful advice. Mr K. G. V. Smith and other members of the Diptera Section of the British Museum of Natural History identified the Diptera prey organisms, Dr W. E. le Quesne the Auchenorhyncha and Dr A. R. Hill the Miridae. I am very grateful to Mr P. S . Meadows for his helpful criticism of the paper.

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