20 AN INTRODUCTION TO ENTOMOLOGY. 



The Tibia. — The tibia id) is the fourth part of the leg. It consists of a sin- 

 gle segment; and is usually a little more slender than the femur, 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 spines 

 which are much larger than the other hairs and spines which arm the leg ; these 

 are called the tibial spines or 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 number of segments in the 

 tarsus is five. The distal segment bears one or two claws (e 1 ). Sometimes 

 these claws are strongly bifid or toothed ; so that a tarsus may appear to bear 

 four or even six claws. The tarsi vary much in form, and thus present charac- 

 ters which are useful in classification. Sexual characters are also frequently 

 presented by this part. 



On the ventral surface of the segments of the tarsus in many insects are 

 cushions of short hairs or of membrane, capable of inflation, or concave plates, 

 which act so as to produce a vacuum and thus enable the insect to walk on the 

 lower surface of objects. These cushions or plates are called pulvilli (e 2 ). In 

 many insects the pulvillus of the distal segment of the tarsus is a circular pad 

 projecting between the tarsal claws. In most descriptive works this is referred 

 to as the pulvillus, even though the other pul villi are well developed. The pul- 

 villi are also called the onychii by some writers. 



With many insects {e.g. most Diptera) the distal segment of the tarsus bears 

 a pair of pulvilli, one beneath each claw. In such cases there is frequently 

 between these pulvilli a third single appendage of similar structure; this is 

 called the empodium. In other insects the empodium is bristle-like or alto- 

 gether wanting. 



The proximal segment of the tarsus is designated in some descriptive works 

 as the metatarsus. 



Tlic Wings. — The normal number of wings is two pairs ; but in 

 addition to the large order Diptera, there are many .insects which 

 have only a single pair ; and many other insects are wingless. As 

 already stated, the first pair of wings is articulated to the meso- 

 thorax ; and the second pair, to the metathorax. When but a single 

 pair of wings is present, it is almost invariably the first pair. 



Each wing is a plate-like or membranous expansion which is first developed 

 as a sac-like projection of the body-wall.* In the course of the formation of 

 the wing, the dorsal and ventral walls of this sac become united throughout 

 the greater part of their extent. There are usually certain lines along which 

 the walls of this sac are thickened. The thickenings of the dorsal and ventral 

 walls are exactly opposed, and together constitute the framework of the wing. 

 These thickened lines are termed the veins or nerves of the wings ; and their 



* The gradual formation of wings can be easily observed in insects with an incomplete 

 metamorphosis. See description of the transformations of the Acridiidae. 



