THE EXTERNAL ANATOMY OF INSECTS 



57 



WP 



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 proximal end of the femur {e. g. Carabidag). But 

 the fact is that in these insects, although the femiu' 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 (6y, 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 

 -Lateral aspect ^^ usually a little more slender than the femur, 

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

 Na^^onia (From Snod- j^ ^^^^ ^^^^^^^ ^^ ^^^^^ -^ ^^^ ^^^^^^^ ^^^ 



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, varying 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, 



Fig. 68. 



Fig. 69. — A leg of Machilis; 

 s, stylus. 



