314 BIOLOGY: GENERAL AND MEDICAL 



than by design, the " screw-worm " appears to 

 prefer animal tissues to other available food 

 for its young. 



The botfly fastens its eggs to the hairs of horses, 

 from which position they are licked off and 

 swallowed, to hatch in the animal's stomach. 

 The larvae attach themselves to the mucous 

 membrane, where they remain until ready to 

 pupate, when they let go, pass through the 

 pylorus, and reach the anus by the time the 

 imago is ready to emerge. 



The sheep bot lays her eggs at the nostrils of 

 the sheep, from which the hatched larvae ascend 

 to the frontal sinuses where their presence causes 

 vertigo and sometimes the death of the sheep. 

 When the larvae are mature they escape from 

 the nose and pupate on the ground. 

 Botflies of the genus Hypoderma sometimes 

 form tumors through irritation of the skin in 

 which the larvae develop. The "ox-warble," 

 like the horse bot, lays its eggs on the skin, 

 from which they are licked and swallowed. 

 The larvae make their way through the oeso- 

 phagus, to the subcutaneous tissue, where they 

 cause tumor-like swellings from which they bore 

 out at maturity, leaving permanent holes in the 

 animal's hide. 



Many biting flies, Stomoxys, Glossina, etc., not 

 only annoy warm-blooded animals by biting 

 them, but act as hosts of parasites which they 

 take from the blood of one animal, and pass to 

 healthy animals subsequently bitten. The 

 discovery that the trypanosomes of "tsetse 

 fly disease *' of animals and "sleeping sickness" 

 of man are spread by the Glossina morsitans 

 and Glossina palpalis, respectively, and that 

 those of "surra" and mal de caderas are simi- 

 larly spread by Stomoxys is of marked economic 



