A laboratory study of optimal behaviour and the costs of net construction by Polycentropus Havomaculatus (Insecta: Trichoptera: Polycentropodidae)

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Abstract

Metabolic rates of Polycentropus flavomaculatus larvae in a flow-through respirometer at 15°C were 17 % higher during net spinning than while larvae were resting. The energy content of net silk and caddisfly tissues, obtained by bomb calorimetry, were 23–367 J mgAFDW-1 and 25–715 J mg AFDW-1, respectively. Total cost of net construction for an ‘average’ 5th instar larva (2–808 mg dry wt) was 0124 J as silk (75–8 % of total cost) and 0030 J as increased activity over the 293-minute building period at 15°C.

A significant cost jn net building was demonstrated by larvae which were forced to rebuild their nets daily. They lost more weight than larvae which were forced to rebuild every second day; undisturbed (starved) larvae lost least weight. The weight of nets which were rebuilt daily declined over successive constructions, but net allocation (net weight/larval weight) remained rather constant. The major energetic expense of undisturbed larvae was routine metabolism, whilst that of the daily rebuilders was silk synthesis.

Feeding regime had a marked influence on net construction, and ad libitum fed larvae built larger nets than those fed every second day; starved larvae built the smallest nets. Food capture was apparently linked to the initiation of construction behaviour and thus a more liberal feeding regime was associated with larger nets.

The energy gain (food intake –costs of metabolism and silk synthesis) of ad libitum fed larvae was greater than that of animals fed every second day, and the cost/benefit function for both groups declined throughout the experiment. While each meal was associated with additional net-spinning costs, over time, the animals tended towards greater energetic economy.

Prior feeding regime had no effect on the weight of nets built subsequently; nets were spun without reference to expectation of prey capture. Prior feeding regime had a significant effect on giving-up time, and those animals with previous experience of high prey capture rates abandoned nets sooner than larvae which had experienced a lower rate of encounter. However, the giving-up time of all treatment groups (< 50 % given-up after 30 days) was longer than previously reported for polycentropodids.

Polycentropus flavomaculatus larvae have resting metabolic rates lower than those of ‘normal’ poikilotherms and spiders, but similar to those of ant lions. These differences are correlated with modes of food capture, and allow P. flavomaculatus to endure periods of food scarcity.

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