Consequences of the instar stage for behavior in a pit-building antlion

1. What they did

Antlion larvae are small insect predators that dig cone-shaped pits in loose sand and wait for ants and other small arthropods to tumble in. Like other insects, they grow through stages called instars, moulting between each. Antlions have three larval instars, and the third is the longest, more than half the larval period. Researchers often match animals by body mass and assume similar-sized individuals are alike. The authors asked whether larvae of the same mass but a different instar behave differently.

They collected larvae of Myrmeleon hyalinus, the most common pit-building antlion in Israel, near Beer-Sheva, and tested them in the laboratory. Before each experiment the larvae were fed, weighed and then starved for seven days so they started in the same condition. They measured four behaviours:

• Pit size: pit diameter.
• Response time to prey: how long a larva took to throw sand at a flour beetle larva placed at the edge of its pit (60 seconds if it did not respond).
• Relocation: how far a larva moved its pit over 48 hours in a larger tray of sand in the dark, and again 24 hours after the researchers wiped out its pit and traces (a disturbance like wind, rain or animals passing).
• Prey exploitation efficiency: the weight a larva gained from one prey item, divided by the weight of the prey.

Experiment 1: 50 larvae of very similar mass (about 6 mg), 29 in the second instar and 21 in the third, told apart by head width and mandible length. They were tested once as they were, and again after the second instar larvae had moulted into the third instar. They also measured head width, mandible length and abdomen width, to see whether body shape predicted behaviour.

Experiment 2: 60 second instar larvae, split into small (early) and large (late) groups. The small ones were tested again after they had been fed and grown.

2. What they found

Same weight, different stage (experiment 1)

• Third instar larvae built bigger pits, responded faster to prey and ate a larger share of each prey item than second instar larvae of almost the same mass.
• These differences disappeared once the second instar larvae had moulted into the third instar.
• Relocation distances did not differ between the stages, but all larvae moved further in the first phase.
• Pit size separated the two stages best, followed by prey exploitation efficiency and relocation distance; response time separated them least.

Consistent individuals (third instar larvae)

• Each larva’s pit size and response time were consistent between two trials.
• Relocation distance after disturbance was consistent, but not before disturbance. Prey exploitation efficiency was not consistent.
• Behaviours fell along two axes. The first linked big pits, fast responses and eating more of each prey item: larvae that invested more in foraging. The second linked slow responses to prey with moving pits further.
• Head width, mandible length and abdomen width did not predict either axis.

Small versus large second instar larvae (experiment 2)

• Small second instar larvae responded faster to prey and moved further after disturbance than large ones. Pit size, prey exploitation and relocation before disturbance did not differ.
• When the same small larvae had grown larger, still in the second instar, their behaviour had not changed significantly.
• So larvae of different stages and similar mass differed more than larvae of the same stage that differed in mass by over 60%.

3. What they concluded

• Animals of similar size, especially those collected in the field, are not necessarily alike. Instar stage predicted antlion behaviour better than body mass within an instar.
• Studies of insect larvae should record instar stage as well as body mass and include it in their design and analysis; ignoring it could make stage differences look like individual personality.
• Third instar larvae, which must store resources for adult reproduction, seem to invest more in foraging, as state-dependent models predict.
• The link between larger larvae and larger pits is probably due to head and mandible size, which differ between instars, rather than total body mass. Within the third instar, body shape had little effect, perhaps because hunger, density, sand type, temperature and experience shape pit size more.
• Larvae remained consistently different even within the same instar. The authors suggest studying physiology, such as metabolic rate, and behaviour across different situations.

6. Other technologies, skills or innovations

• Instar stage identified from a plot of head width against mandible length, measured from stereoscope photographs with the free program ImageJ.
• Simple laboratory measures of foraging: pit diameter with a digital caliper, response time to a standard prey item, and weight gain from one prey item on an analytical scale.