When Drosophila see parasitoid wasps, they mate faster.
Not metaphorically. Ebrahim, Talross, and Carlson (Nature Communications, 2021) exposed fruit flies to the sight of Leptopilina wasps — parasitoids whose larvae develop inside Drosophila larvae, consuming the host from within. Copulation latency dropped significantly (p < 0.0001). The effect was driven by female behavior: females became receptive sooner. It was visual: blocking specific visual neurons in the fly brain eliminated the response. And it was species-specific: only wasps that actually parasitize Drosophila triggered it. Unrelated wasp species did not.
The molecular mechanism is a single gene. Visual contact with wasps induces a 20-fold upregulation of IBIN in the nervous system — eyes, optic lobes, and other brain regions. IBIN encodes a micropeptide of 41 amino acids. Mutant flies lacking functional IBIN do not accelerate mating when exposed to wasps. The entire behavioral cascade — predator detection, threat assessment, reproductive acceleration — runs through a molecule smaller than most protein domains.
The standard defensive repertoire against parasitoids includes behavioral avoidance (females lay fewer eggs on wasp-contaminated substrates), immune preparation (upregulating blood cell production), and larval resistance (encapsulating parasitoid eggs in melanin). These are all defenses aimed at survival. The mating acceleration is different. It does not increase the probability of surviving the threat. It increases the probability of having reproduced before the threat arrives.
The response persists for at least 24 hours after wasp exposure. The fly does not need to keep seeing the wasp. A single visual encounter reprograms reproductive urgency for a day.
The distinction matters. Most documented anti-predator responses are either evasive (fleeing, hiding) or preparatory (immune priming, defensive morphology). Both assume the organism will survive the encounter and benefit from having prepared. The mating acceleration assumes the opposite. It treats the wasp's presence as information about reduced life expectancy and converts that information directly into reproductive output. The strategy is not “survive the threat” but “reproduce before it arrives.”
This is terminal investment compressed into an insect timescale. In vertebrate ecology, terminal investment describes the increased reproductive effort of organisms approaching death — older animals mating more aggressively, parasitized fish spawning earlier. The mechanism in vertebrates is typically hormonal, slow, and systemic. In Drosophila, the mechanism is a micropeptide triggered by a visual circuit, acting within hours. The concept is the same. The engineering is faster by orders of magnitude.
The 41-amino-acid molecule sits at the boundary between perception and reproduction. It is too small to be a conventional signaling protein, too specific to be accidental. It translates one channel — “I see a wasp” — into another channel — “mate now” — with no intermediate deliberation. The fly does not weigh options. The molecule converts threat into urgency directly, as a circuit, not a decision.