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Survival strategies and cost-benefit-balances of the predatory insect larva (Euroleon nostras FOURCROY, Myrmeleontidae)

We are investigating the behavior strategies and the evolutionary adaptations and we are also determining the quantitative parameters for cost-benefit-balance in the antlions Euroleon nostras, Myrmeleon formicarius and M bore. Antlions live in sand mostly in deserts and semideserts or in zones of glacial origin and in coastal dunes. The probability of prey capture is increased to about 200 species as they construct pit. However: larvae that construct traps remain essentially inactive at the ambush-site and wait for active prey.

Larvae of E. nostras need a very short time (approximately 25 min) for constructing a pitfall trap in relation to the time at resting metabolic rate. Therefore, the costs of the pit construction (volume, mass weight, energy) are of minor importance in relation to prey. The linear accession of pit traps diameter (benefit of prey catch) exponentially corresponds to ascent of costs. Although ants live in aggregations and use paths, the capture rate of an antlion remains very low, because prey is arriving seldom and in unpredictable intervals. In one of the survey habitats with 48 pits the antlions captured only 17 prey in 4 days. Each pit results in a capture rate of 1 prey/11.3 d. Capturing rates of 1 prey/3.3 d and 1 prey/24.0 d respectively were reported for 2 other areas at the same time.

The body masses of the first, second and third larval stages varied from 39 to 50% of the means (2.71 +/- 1.22 mg, n = 34; 15.20 +/- 7.57 mg, n = 34; 57.9 +/- 22.22 mg, n = 29). For all the larvae instars the mean extraction rate of living mass of an ant was 55 to 70% of the live weight. The surviving of hunger periods up to more then 4 months of the first and second larval instars and up to 8 months in case of the third instar is part of evolution-conditioned adjustments. An antlion pit is built within 25 min activity time. The function time of the pit was about 23:35 h within a 24 h-day before beginning of a pit enlargement activity phase.

The pit construction requires minimum energetic costs, whereas the gain obtained by capturing a prey amounts to 20500 mJ per single prey. The calculation is based on the energy content of investigated food and the carbon content of the prey Formica spec. Our results indicate that an antlion with a wet weight (ww) of 22.6 mg releases 0.62 mu g C*h(-1) (1.3 J*g(-1) ww *h(-1)) within periods of inactivity and 5.28 mu g C*h(-1) (11.6 J*g(-1) ww*h(-1)) during construction of a pit (rest : activity 1:8,5). Considering an estimated extraction efficiency of 36.5% at feeding on Formica, one prey of this kind compensates the loss of carbon or energy originated by the resting metabolic rate of the larvae for 25 days.