Giant Ichneumon Wasp

Family Ichneumonidae — the ichneumon wasps — contains over 25,000 described species and is widely considered the most species-rich animal family known to science (estimates suggest over 100,000 species exist; most are still undescribed). All are parasitoid wasps: females lay eggs on, in, or near a host arthropod (most often the larva or pupa of another insect), and the wasp larva develops by feeding on the host's tissue, eventually killing it. Megarhyssa macrurus is one of the most spectacular ichneumonids in North America: females reach 4 cm body length and 12 cm ovipositor length, and they hunt for the larvae of pigeon horntail wasps (Tremex columba) that bore tunnels in dead and dying hardwood trees. The female ichneumon detects the host larva by sensing the vibrations of its mandibles chewing wood and by detecting fungal compounds the horntail uses to soften the wood; she then balances on the bark with her ovipositor extended vertically and slowly drills through 5-10 cm of wood (a process taking 30-60 minutes) until the ovipositor tip reaches the host. She deposits a single egg on the horntail larva. The ichneumon larva hatches and eats the horntail from the inside out, killing it before pupating in the same tunnel and emerging the following spring as an adult. The ichneumon-host relationship is one of the most dramatic examples of parasitoid biology and was famously cited by Charles Darwin (in an 1860 letter to Asa Gray) as the example that hardened his rejection of natural-theology arguments for benevolent design in nature: 'I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.'