Trochantin

Definition

A small, often triangular or wedge-shaped sclerite of the insect leg that articulates between the coxa and the trochanter, sometimes fused to either element. In the generalized insect leg, the trochantin forms part of the pleural wall and serves as the anterior articular point for the coxa, permitting leg movement. Its presence, reduction, or fusion is taxonomically informative: it is well-developed and freely articulated in many basal insect orders (e.g., Odonata, Orthoptera) but reduced or fused to the coxa in higher groups such as Coleoptera and Diptera. In some contexts, arachnologists use the term more loosely for proximal leg articles.

Etymology

From New Latin, diminutive of trochanter, Greek trokhanter (runner, from trekhein, to run), with the diminutive suffix -in indicating a smaller associated structure.

Example

In a grasshopper (Orthoptera), the trochantin is visible as a distinct, movable sclerite at the base of the leg, separated from the coxa by a membrane; in a beetle (Coleoptera), the trochantin is typically fused immovably to the coxa and appears only as a suture or is entirely obliterated.

Synonyms

• trochantine (variant spelling)

Related Terms

• Trochanter
• Coxa
• Femur
• pleuron
• sternum
• subcoxa
• meron
• precoxa

Usage Notes

The trochantin is frequently overlooked in routine identification because it is small and often fused. When describing leg structure, specialists distinguish between a 'free trochantin' (movably articulated) and a 'fused trochantin' (sclerotized to the coxa). The term should not be confused with 'trochanter,' the much larger segment immediately distal to it. In some older literature, 'trochantin' was applied to structures now recognized as the subcoxa or meron. When in doubt, dissection and examination of the articular membrane can confirm whether a sclerite is a true trochantin.