Planidium

Definition

A highly modified, mobile first-instar larva of certain parasitoid insects, characterized by strong sclerotization, a flattened or compact body, and active host-seeking behavior. Unlike typical insect larvae that remain near the oviposition site, the planidium functions as a dispersal stage that actively locates and penetrates a host (often another arthropod) before transforming into a less mobile, feeding larval form.

Etymology

From Greek planēs 'wanderer' + diminutive suffix -idium, referring to the active, wandering habit of this larval stage.

Example

In the parasitoid wasp family Eucharitidae, the planidium hatches from an egg laid on vegetation, drops to the ground, and actively searches for foraging ants to attach to; once carried into the nest, it transfers to ant brood and becomes an endoparasitic larva.

Related Terms

• triungulin
• Hypermetamorphosis
• first instar
• Parasitoid
• Sclerotization
• Phoresy

Usage Notes

Planidia are distinct from typical triungulins (found in Strepsiptera and some beetles) in their degree of sclerotization and specific association with parasitoid life histories; the term is not used for mobile larvae in non-parasitoid contexts. Some authors restrict 'planidium' to Eucharitidae and Perilampidae, while others apply it more broadly to any hardened, mobile first instar with host-seeking function. Contrast with 'campodeiform' larvae, which are active and predatory but not necessarily sclerotized or parasitoid-specific.