THE EXTERNAL ANATOMY OF INSECTS 



57 



Thysanura, the coxa of each middle and hind leg bears a small 

 appendage, the stylus (Fig. 69). The styli are of great interest as 

 they are believed to correspond to one of the two branches of the legs 

 of Crustacea; thus indicating that insects have descended from 

 forms in which the legs were biramous. 



In several genera of the Thysanura one or more of the abdominal 

 segments bear each a pair of styli ; in Machilis they are found on the 

 second to the ninth abdominal segments. These styli are regarded as 

 vestiges of abdominal legs. 



The trochanter. — The trochanter is the second part of the leg. It 

 consists usually of a very short, triangular or quadrangular segment, 

 between the coxa and the femur. Sometimes the femur appears to 

 articulate directly with the coxa ; and the trochanter to be merely an 

 appendage of the prox'mal end of the femur {e. g. Carabidas). But 

 the fact is that in these insects, although the femur may touch the 

 coxa, it does not articulate with it; and the 

 organs that pass from the cavity of the coxa 

 to that of the femur must pass through the 

 trochanter. In some Hymenoptera the tro- 

 chanter consists of two segments (67, B). 



The femur. — The femur is the third part of 

 the leg; and is usually the largest part. It 

 consists of a single segment. 



The tibia. — The tibia is the fourth part of 

 the leg. It consists of a single segment; and 

 Fig. 68.— Lateral aspect ^^ usually a little more slender than the femur, 

 of the mesothorax of although it often equals or exceeds it in length. 

 In such species as burrow in the ground, the 

 distal extremity is greatly broadened and 

 shaped more or less like a hand. Near the distal end of the tibia 

 there are in most insects one or more spurs, which are much larger 

 than the hairs and spines which arm the 

 leg; these are called the tibial spurs, and 

 are much used in classification. 



The tarsus. — The tarsus is the fifth and 

 most distal part of the leg, that which is 

 popularly called the foot. It consists of a 

 series of segments, var^dng in number 

 from one to six. The most common num- 

 ber of segments in the tarsus is five. 



In many insects, the first segment of the tarsus is much longer, 



Neuronia (From Snod- 

 grass). 



Fig. 69. — A leg of Machilis; 

 s, stylus. 



