THE BRIEFLY COLONIAL LIFE OF HATCHLINGS OF THE NET-SPINNING CADDISFLY PLECTROCNEMIA-CONSPERSA

Egg masses of the European net-spinning caddisfly Plectrocnemia conspersa were collected from a small stream, with the object of studying egg incubation and early (first seven days) larval behaviour. Egg masses contained 150-800 eggs (mean 440.2, 95% CL 75.6). Egg incubation period in the laboratory (y days) was related to temperature (x-degrees-C) by the equation ln(y) = 3.14 + 0.144 ln(x), taking 93 days at 6-degrees-C and 20 days at 18-degrees-C. Egg masses incubated in the field hatched as predicted by this equation. The length of individual eggs increased by an average of 17% during incubation. Emergence from the egg took about 30 min, and newly hatched larvae had yolk in their guts which lasted 24-30 h, after which feeding began. Larvae took only live prey. In still water, larvae span silk, ventilated using abdominal undulations, turned in the net, and performed further miscellaneous movements. Newly hatched 'yolky' larvae spent the most time spinning; older larvae resident in the mass of silk near the maternal egg mass carried out most ventilations and turns, and nonresident larvae made most other, miscellaneous movements. Interactions among siblings were categorised as 'aggressive', 'ignore' or 'explore'. Interactions most commonly involved older residents, which were the only larvae to show aggression. Overall, most interactions were 'ignore' and 'explore'. In running water a large single net (up to 10 cm2), containing > 300 larvae, was formed around one old egg mass. This colonial web persisted for at least 7 d and, although density declined rapidly, 3-4 larvae/cm2 remained after 7 d. This pattern was repeated in three other egg masses kept in running water, net size reaching a peak approximately 3 d after the onset of egg hatching. Behaviour of individual larvae in running water was similar to that in still water, except that in flowing water they did not ventilate and turning behaviour was less frequent. During the first three days of life, residents in the net in flowing water ignored or explored each other without aggression. The first aggressive interactions were observed on day 4. It is proposed that the sharing of silk is the main selective value of this briefly colonial system. Silk provides shelter and probably increases the likelihood of obtaining the first meal.