32 



THE STUDY OF INSECTS 



Fig. 64. — Legs of insects: A, wasp; B, ichneu- 

 mon-!ly; C, bee; c, coxa; tr, trochanter; /, femur; 

 /;', tibia; la, tarsus; met., metatarsus. 



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

 are cushion-like structures termed pulvilli. In many insects the pulvilli 

 of the last segment of the tarsus are circular pads beneath the tarsal 

 claws. In most descriptive works these are referred to as the pulvilli, 

 even though the other pulvilli are well developed. 



The pulvilli of some insects, notably those of the Diptera, bear fine, 



hollow hairs, called tenent hairs, from 

 which an adhesive fluid exudes that 

 enables the insect to walk on the 

 undersides of objects. 



The wings. — The two pairs of 

 wings are borne by the mesothorax 

 and metathorax, but either or both 

 pairs may be wanting. Thus the 

 flies, or Diptera, have only the first 

 pair of wings fitted for flight, the 

 second pair being represented by a 

 pair of knobbed threads; and with 

 the earwigs and beetles each of the 

 first pair of wings is hard and to- 

 gether they form a cover for the hind 

 pair. 



In form an insect's wing is a large, 

 membranous, leaf-like appendage, 

 which is thickened along certain lines. These thickened lines are termed 

 the veins or nerves of the wing; and their arrangement is described as the 

 venation or neuration of the wings. The thin spaces of the wings which 

 are bounded by the veins are called cells. When a cell is completely 

 surrounded by veins it is said to be closed; but when it extends to the 

 margin of the wing it is said to be open. 



The wings of different insects vary greatly in structure, and thus 

 afford excellent distinctions for the 

 purposes of classification. The vari- 

 ous parts of the wing have, there- 

 fore, received special names. There 

 is considerable lack of uniformity 

 among entomologists as to the names 

 applied to these parts; but we have 

 adopted the set of terms defined be- 

 low as representing the best usage. 



An insect's wing is more or less 

 triangular in outline; it therefore 

 presents three margins: the costal 

 margin, or costa (Fig. 65, a-b); the 

 outer margin (Fig. 65, b-c); and the inner margin (Fig. 65, c-d). 



The angle at the base of the costal margin (Fig. 65, a) is the humeral 

 angle; that between the costal margin and the outer margin (Fig. 65, b) 

 is the apex of the wing; and the angle between the outer margin and 

 the inner margin (Fig. 65, c) is the anal angle. 



There have been many different sets of names applied to the veins of 

 the wings. Not only have the students of each order of insects had a 

 peculiar nomenclature, but in many cases different students of the same 



Fig. 65. 



angles. 



Diagram of a wing showing margins and 



