MECHANICS OF WALKING 



103 



Hepialus hectus are shortened. In an aphid (Mastopoda pteridis, 

 CEsl.) all the tarsi are reduced to a single vestigial joint (Fig. 114). 



Entirely legless adult insects are rare, 

 and the loss is clearly seen to be an adapta- 

 tion due to disuse ; such are the females of 

 the Psychidae, the females of several 

 genera of Coccidse (Mytilaspis, etc.), and 

 the females of the Stylopidae. 



Apodous larval insects are common, and 

 the loss of legs is plainly seen to be a 

 secondary adaptive feature, since there are 

 annectant forms with one or two pairs of 

 thoracic legs. All dipterous and siphonap- 

 terous larvae, those of all the Hymenoptera 

 except the sawflies, a few lepidopterous 

 larvae, some coleopterous, as those of the 

 Rhyncophora, Buprestidse, Eucnemidse, and 

 other families, and many Cerambycidee are 

 without any legs. In EupsaUs minuta, 

 belonging to the Brenthidae, the thoracic 

 legs are minute. 



The legs of larvae end in a single claw, upon the tips of which the 

 insect stands in walking. 



sh- 



FIG. 1 13. Leg- of Litognatka : 

 cee, coxa; /, femur; I, tibia; < : j>, 

 its epiphysis, and .sA, its shield-like 

 process. "The tarsus entirely want- 

 ing. After Smith. 



c. Locomotion (walking, climbing, and swimming) 



Mechanics of walking. --To Graber we owe the best exposition of 

 the mechanics of walking in insects. 



"The first segment of the insect leg," he says, "upon which the weight of 

 the body rests first of all, is the coxa. Its method of articulation is very differ- 

 ent from that of the other joints. The enartlirosis affords the most extensive 

 play, particularly in the Hymenoptera and Diptera." 



In the former the development of their social conditions is very closely con- 

 nected with the freest possible use of the legs, which serve as hands. In the 

 beetles, however, which are very compactly built, there exists a solid articula- 

 tion whereby the entire hip rests in a tent-like excavation of the thorax, and can 

 only be turned round a single axis, as may be seen in Fig. 115, where c repre- 

 sents the imaginary revolving axis and d the coxa. In the case we are suppos- 

 ing, therefore, only a backward and forward movement of the coxa is possible, 

 the extent of the play of which depends on the size of the coxal pan, as well 

 as certain groin or bar-like structures which limit further rotation. In the very 

 dissimilar arrangement which draws in the fore, middle, and hind legs toward 

 the body it is self-evident that their extent of action is also different. This 

 arrangement seems to be most yielding on the fore legs, where the hips, to con- 

 fine ourselves to the stag-beetles, can be turned backward and forward 60 from 



