Trochanter

Definition

The second segment of an arthropod leg, located between the coxa (the basal segment articulating with the body) and the femur. In insects, the trochanter is typically small and immovably fused to the femur in most groups, though it remains distinct and freely articulating in some basal lineages such as Odonata and Orthoptera. In arachnids and many crustaceans, the trochanter is more prominent and independently mobile, serving as a functional hinge that increases leg flexibility. The term also applies to analogous segments in myriapods and some other arthropods.

Etymology

From Greek trochanter, 'a runner', via New Latin, originally applied to the bony protuberance on the mammalian femur before adoption into entomological anatomy.

Example

In a grasshopper (Orthoptera), the trochanter is clearly visible as a distinct segment between the coxa and femur, allowing greater range of motion for jumping; in contrast, a honey bee (Hymenoptera) shows the trochanter fused to the femur, appearing as a slight constriction rather than a separate segment.

Related Terms

• coxa
• femur
• Tibia
• tarsus
• Pretarsus
• Arthropod leg segmentation
• Subcoxa

Usage Notes

In insect identification, the degree of fusion between trochanter and femur can be diagnostically important; some keys distinguish 'trochanter+femur' as a functional unit when the suture is obliterated. The term should not be confused with the mammalian anatomical usage (bony protuberance on the femur), though both derive from the same root. When describing leg segments from proximal to distal, the standard sequence is: coxa, trochanter, femur, tibia, tarsus, pretarsus.