THE THORACIC APPENDAGES 



25 



t 



a small segment only, but, with the development of the hexapodous 

 condition, it acquires greater relative size and importance. In 

 the Symphyla and other of the lower arthropods the femur is 

 movable by muscles originating within the trochanter or coxa. 

 Among the Insecta it is usually only provided with a reductor 

 muscle whose origin is in the trochanter : in cases where there 

 is no movement between the femur and trochanter this muscle is 

 undeveloped. 



The homologies of the next three leg-segments, viz., trochanter, 

 coxa and subcoxa, have been productive 

 of much diversity of opinion. In some 

 insects, e.g., MachiUs, certain Coleopterous 

 larvae such as Dytiscus and Philonthus 

 (Fig. 12), together with the nymphs and 

 imagines of Odonata (Fig. 13, C), the 

 trochanters are double. The condition in 

 the Odonata has been investigated by 

 Snodgrass (1927), who has pointed out 

 that the reductor femoris muscle arises in 

 the second trochanter and is attached to 

 the base of the femur. It is evident, 

 therefore, that the first trochanter is with- 

 out muscles and simply functions as the 

 articulatory sclerite between the coxa and 

 the second trochanter. Hansen has shown 

 that the reductor femoris muscle is similarly 

 located in the second trochanter in the larva 

 of Dytiscus. The few writers who have 



studied these parts, viz., Verhoeff (1903), Snodgrass (1927) and 

 Hansen (1930), see in the divided trochanters a primitive condition. 

 Hansen homologises them with the preischium and ischium of 

 such Crustacea as the Pericarda and Anaspidacea. He further 

 comments upon the fact that these two parts are fused in the 

 majority of the Insecta, as in most Decapod Crustacea. In view 

 of the remoteness of the affinity between insects and crustaceans, 

 Hansen's conclusions are unlikely to fyid general acceptance. It 

 is extremely unlikely that preischium and ischium would retain 



Fig. 12. A. Dytiscus 

 sp. larva. Part of 

 first left leg showing 

 divided trochanter, t. 

 B. Philonthus ceneus 

 larva. Part of third 

 left leg showing 

 divided trochanter, t. 

 c, apex of coxa. (From 

 Hansen, 1930.) 



