THE FLIES: DIPTERA 61 



The larva then attaches itself to the lining of the stomach 

 by means of hooks, which encircle the mouth, and for nearly 

 a year feeds on the substance of the stomach-wall. The pupal 

 stage is passed in the earth, which is reached through the 

 alimentary canal. A few of these parasites do 110 particular 

 harm, but a large number may cause death. 



The sheep bot-fly ( (Es'trus o'vis) in a similar manner attacks 

 the nostrils of sheep. The appearance of one of these flies 

 in a flock of sheep is sufficient to throw them into a state 

 of panic ; they run about with their noses between their 

 legs, or try to bury them in dust, to escape their tormentors. 

 The female bot-fly does not lay her eggs as do most other 

 insects, but retains them within her body till they hatch, 

 and then deposits the living young, that is to say, she is 

 viviparous. 



Hover-Flies. Often a collector captures an insect flying 

 about flowers, which has the characteristic manner and yellow 

 and black colors of a wasp, but which has only two wings. 

 It is one of the hover-flies, many of which afford illustrations 

 of protective mimicry. Some species of Eris'talis (Fig. 45) 

 mimic the male honey-bee, and are therefore named drone- 

 flies ; others belonging to Volucel'la (Fig. 45) mimic bumble- 

 bees. The larvae of some feed upon aphids, and are therefore 

 beneficial to the farmer ; others inhabit pools of stagnant 

 water or decaying wood. Several are called rat-tailed larvae, 

 from the presence of a long tube projecting from the pos- 

 terior end of the body, which enables the larva to get air 

 when submerged. The larva of the drone-fly is of this type. 

 Other species are found in ants' nests. 



Mosquitoes. The mosquitoes are a group widely distributed 

 over the tropical and temperate regions of both hemispheres. 

 In nearly all the species observed the mouth-parts of the 

 females only are fitted for piercing the skin of animals. 

 The males, if they feed at all, probably suck the fluids of 



