173 



the skin. They are all the larvae or early states of two-winged flies (Diptera) belong- 

 ing to the family (Estridse, characterized by having the mouth parts entirely obsolete, 

 and popularly called gad-dies or bot-flies. ** * * In the third kind, the parent 

 lays the egg on those parts of the body which can not well be reached by the mouth 

 of the animal attacked, and the young grub, which soon hatches, burrows in the 

 flesh, and subsists upon pus and the diseased matter which results from the wound 

 inflicted, and the irritation is constantly kept up. The well-known wormal or ox 

 bot {Hypoderma bovis), so common along the backs of our cattle, and especially of 

 yearlings and two-year-olds, and dreaded as much by the tanner as by the animal in- 

 fested, is typical of this kind. Residing in a flxed spot, we no longer find in this 

 species the strong hooks at the head, and the spines around the body are sparse and 

 very minute, while the parts of the mouth are soft and fleshy. 



All bot-larvaB breathe principally through two spiracles placed at the blunt and 

 squarely-docked end of the body, and in the ox bot these are very large, and com- 

 pletely fill up the hole to the tumor in which the animal dwells. When ready to 

 transform, it backs out of its residence, drops, and burrows into the ground, and 

 there, like the other species, contracts and undergoes its final change to the fly. The 

 eggs of this ox bot are elliptic-ovoid, slightly compressed, and have at the base a 

 five-ribbed cap on a stout stalk with which to strongly attach them to the skin of the 

 animal. (See Fig. 33a.) 



The perfect insect (see Fig. 31) is something over one-half inch in 

 length, black, banded with yellow, as in- 

 dicated in the figure, and is not unlike 

 a bee in appearance. The flies issue 

 during the entire summer, but are par- 

 ticularly abundant during the months 

 of July and August. The individual life 

 of each fly is, however, comparatively 

 brief, not exceeding a month. The time 

 between the deposition of the egg and 

 its hatching has not been definitely 

 observed, but, from what is known of 

 other species of the family, will be 

 found to last but a few days. Dur- 

 ing the fall and winter months the young larvse develop very slowly ; 

 but in spring and early summer growth is much more rapid and the 

 characteristic hard swellings with central opening, now large and prom- 

 inent, exuding a yellowish matter, may easily be discovered. Fig. 33 & 

 represents the full-grown larva, together with the fig- 

 ures of the anal breathiug pores, all enlarged. (The 

 lines at the side of the larva, puparium, and egg indi- 

 cate their natural size.) 



On escaping from the back of the animal the larva, 

 TiG.Z2.-Hypoderma wn i c h \ n the earlier stages is yellowish white, is of a 



bovis ; head of female ° ^ 



fly from the front— en- gray color, which rapidly darkens until in the con- 

 larged (after Brauer). tracted puparium the color becomes very dark brown, 

 almost black. The pupa state lasts about thirty days, the time depend- 

 ing somewhat upon the weather, and the perfect insect escapes by forcing 

 open a peculiar subtriangular lid at the anterior extremity of the pupa- 



FlG. 31.— Hypoderma bovis— enlarged 

 (after Brauer). 



