252 SHORT COMMUNICATIONS

E. D. M. MACAULAY1): .4 s imple insect 1light recorder.

Much of the work on insect flight durat ion has been with tethered insects. Krogh & Weis- Fogh (1952) devised a flight mill to study locust flight, and this principle was developed (Kishaba et al., 1967) to record the flight of moths automatically. Such flight mills are reasonably cheap and are useful in studies on fuel and respiratory metabolism (Yurkiewicz, !967), but the insects must be at tached to the mill and in the handling may be damaged or their flight behaviour altered. The insect is also compelled to fly abnormally in a circular path, and its flight attitude may be unnatural and may affect performance.

Kennedy & Booth's (1963) flight chamber is probably the best method of studying flight under nearly natural conditions but large powerful insects cannot be f lown in it.

Several aktographs have been described, based on the capacitance changes a moving insect produces in a special cage, (Grobbelaar et al., 1967).

Edwards (1960) used the electric charge on a flying insect to measure the flight activity of mosquitoes, and Baker (1970) developed a system of A. F. amplifiers to record the total activity of caged insects. These systems are expensive and Baker's does not distinguish between flight and crawling.

The apparatus described in this paper records the onset and durat ion of flight and was used successfully with the moth Plusia g a m m a L.

Fig. 1 is a simplified block diagram of the ten-channel flight recorder. The sensor is a d.c. resistance bridge (Fig. 2) powered by a 12v supply consisting of a t ransformer and bridge rectifier. Two arms of the resistance bridge are bead thermistors, one of which (TH1) is placed in the cage containing the moth, the other (TH2) compensates for changes in ambient temperature. R1 and R2 are limiting: resistors to keep the current within the dissipation capabilities of TH1 and TH2. VR1 is a variable resistor for adjusting the current in the bridge; a 5K mult i - turn potent iometer was chosen, because, wired in this way, it provides more sensitive control than the normal pat tern of potentiometer.

All these components are assembled on a 35-ram square of Veroboard and enclosed in a small plastic box. TH1 protrudes through a hole in this box and the temperature-sensitive bead is placed at the centre of the flight cages (Fig. 3), which were t ransparent acrylic food canisters measuring approximately 10 cm X 10 cm • 14 cm high. While the insect is at rest in this cage the self-heating characteristic of the thermistor establishes across the box a thermal gradient that is disturbed when the moth flies. A temperature differential is then established between TH1 and TH2 and a small current flows across the bridge. The problem then is to use this to actuate a recorder.

In the prototype the current was measured as about 200 mieroamperes, which would drive a transistorised relay, but meter relays based on sensitive microammeters whose pointer is the moving contact of a changeover relay are simpler, f in some countries they can be bought cheaply from Government Surplus Depots). Their normally closed contacts can be used to operate the pens of a Miniscript ten-channel event recorder.

The apparatus is simple to operate. A jack socket is provided in the leads to each meter relay, a 200 /z amp test meter is plugged into this socket and the appropriate sensor is connecte~t to its relay; the current is adjusted with VR1 to a value of about 100 # amp. This procedure is repeated for each sensor and the whole apparatus left for a few minutes to equilibrate. After this, without using the test meter, VR1 can be adjusted until the relay and pens are at their threshold, when they will make a mark as soon as the moth flies. This threshold value for each relay can be read with the test meter and used in future operation.

Fig. 4 shows a section of a typical record, in which flight activity is recorded as a

1) Depar tment of Entomology, Rothamsted Experimental Station, Harpenden, Hertfordshire, England.

SHORT COMMUNICATIONS 253

1 Event Recorder

Fig. I. Block diagram of activity recorder.

12v.d.c. Power

Supply

Fig. 2. Thermistor bridge circuit. TH1, TH2, Thermistors TH-B12 (Radiospares), VR1, 5K Linipot (Radiospares) R1, R2, R3, 1K 1/2 W

carbon.

Thermistor

, . / ~ Box containing bridge

_ To power supply & meter relay

Food dish

Fig. 3. Flight cage. The whole cage is lifted to replace the food dish without disturbing

the insect. Fig. 4. Sample of record for a

period of about 9 hrs.

254 SHORT COMMUNICATIONS

displacement of the line towards the perforatod edge of the paper strip. An acceptably accurate estimate of the duration of flights can be made from these displacements as the paper passes the recording styli at 20 cm/hour.

BAKER, C. R. B. (1970). Apparatus for recording flight activity of caged moths. Lab. Pract.

19 : 293. EOWAROS, D. K. (1960). A method for continuous determination of displacement activity in a

group of flying insects. Can. J. Zool. 38 : 1021--1025. GROBBELAAR, J. H., MORRISON, G. J., BAART, E. E. & MORAN, V. C. (1967). A versatile, highly

sensitive activity recorder for insects. J. Insect Physiol. 13 : 1843--1848. KENNEDY, J. S. & BOOTH, C. O. (1963). Free flight of aphids in the laboratory. J. exp. Biol.

4 0 : 67--85. KISHABA, A. N., HENNEBERRY, T. J., HANCOCK, P. J. & TOBA, H. H. (1967). Laboratory tech-

nique for studying flight of cabbage looper moths and the effects of age, sex, food and Tepa on flight characteristics. I. econ. Ent. 60: 359--366.

KROGn, A. & WEIS-FoGH, T. (1952). A roundabout for studying sustained flight of locusts. .l. exp. Biol. 29: 211--219.

YU~IEWlCZ, W. J. (1967). A respirometer flask for measuring oxygen consumption during flight on a turnabout. Ann. ent. Soc. Am. 60 : 1122--1123,

F. MORIARTY1): Dry and [resh weights of Lepidopterous larvae.

Bullock & Smith (1971) found that several species of Lepidopterous larvae have a linear relationship between the logarithms of their fresh and dry weights. Moreover, the slopes of these linear regressions are identical. The percentage dry matter increased as larvae matured, which contrasts with the fact that the hea~ier species had a higher water content than the lighter species throughout their life.

I have used some data from fifth-instar larvae of the nymphalid small tortoise-shell butterfly, AgIais urticae L., to see whether the same simple mathematical relationship exists for this species.

Larvae of .4. urticae were reared from eggs in a constant-temperature room at 25 +- 1 ~ C. and were fed on fresh-cut nettles (Urtica dioica L.). Humidity in the room was not controlled, and ranged between 55 and 70%. Samples of fifth-instar larvae of similar weights were taken at intervals from a uniform group of larvae, weighed, and their dry weight obtained by freeze drying.

There is a distinctly curvitinear relationship between the logarithms of the larval fresh (M and dry (y) weights (Fig. 1). A second degree polynomial was calculated for the data:

log y = 1.3408 - - 3.3017 (log x) + 1.0762 ( log x) 2

This regression has significantly less residual variation than does the linear regression (Fa,7 = 49.7, P < 0.001).

There appears to be a simpler relationship between fresh and dry weights for the species studied by Bullock & Smith (1971) than for A. urticae. However, the individual estimates for A. urticae all fit very closely to the eurvilinear regression, and the amount of curvature is

1) The Nature Conservancy, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, England