Mantidfly larvae use substrate-borne cues to locate and distinguish different sexes and life stages of potential spider hosts

In many animals, early-life decisions influence long-term fitness. Mantidflies are spider egg predators; their tiny larvae climb aboard spiders to find eggs, but little is known about how they find spiders. We tested the hypothesis that mantidfly larvae (Dicromantispa sayi) detect and respond to substrate-borne spider cues (e.g., silk and/or excreta). We presented larvae with filter paper exposed to different types of spiders (adult female, adult male, or juvenile Habronattus trimaculatus jumping spiders) versus a no-cue control. Larvae spent more time on filter paper with spider cues. We then tested the hypothesis that mantidflies make finer distinctions between spiders when given direct choices between these cues. Larvae did not discriminate between sexes but spent more time (and exhibited more phoretic behavior) on filter paper with female or male cues compared with juvenile cues. While this suggests that mantidflies actively seek out adult spiders, we also found that adult spiders were more active than juveniles and may have simply deposited more silk and excreta, providing a stronger cue to detect. We discuss these findings in the context of the risks and benefits of different spider hosts, and how early-life spider-searching strategies may shape a mantidfly’s long-term fitness.

Methods
Data for behavioral experiment with mantispid larvae. Larvae were given a series of choice tests between filter paper halves treated with different cues. Time spent on each filter paper type (1,2) and frequency of phoresy behavior (2) were collected. We also measured spider activity when treating the filter paper with cues as a proxy for how much silk and excreta the spiders laid on filter paper (2). Exploratory analyses using the side the mantispid started on is included in a separate dataset (3).