BULLETIN 67, UNITED STATES NATIONAL MUSEUM. 



legs, and the mesothorax and metathorax ordinarily have each a pair 

 of wings. Behind the thorax is the abdomen, bearing at its tip the 

 genitalia, sting, ovipositor, or sometimes jointed setae or cerci. 



Insects are bisexual, there being a male and a female form. In 

 some social insects there are other forms known as workers and sol- 

 diers. The female commonly deposits eggs, but in some cases living 

 larvae. The eggs hatch into larvae ; the larvae feed, molt, or change their 

 skin several times, and then turn into nymphs or into pupae. The 

 nymph is active and feeds in a similar manner to the larva, but shows 

 small wing-pads on each side of the thorax, the rudiments of the 

 future wings. From the nymph there issues in time the winged 

 insect. This development through the nymphal stage is called an 

 incomplete metamorphosis. Most insects, however, change from the 



larva to the pupa, 

 which is quiescent and 

 does not feed, and dif- 

 fers greatly from the 

 larva in appearance 

 (fig. 2). This pupa 

 may be naked, as a 

 chrysalis, or inclosed 

 within a cocoon of silk 

 or earth. From the 

 pupa there issues in 

 due time the winged 

 insect. This line of 

 m? fr development is called 



Fig. 2.— Diabrotica 12-punctata, showing transformations: a, complete meiamor- 



beetle; b, egg; c, larva; d, end of larva; e, work of larva; phosis. These differ- 

 ences above noted are 

 not only external. In the case of a complete metamorphosis, the 

 changes that occur inside of the pupa are often revolutionary in the 

 extreme. The entire internal structure, except the nervous system, 

 may be reduced to a mass of liquid, and the organs of the mature 

 insect developed from this liquid. 



The wings of the anterior pair in insects are called the forewings or 

 primaries, and the others the hind pair, or secondaries. When the 

 forewings are thickened, they are called tegmina or elytra. The 

 wings have numerous veins through them, the arrangement of which 

 is constant for each species and of great value in classification. This 

 arrangement of these veins is called the venation. The legs are 

 known as front, middle, and hind legs, and each is composed of sev- 

 eral parts. The most basal part and attached to the body is the 

 coxa; the next is a minute piece, the trochanter; the third a long 



