The Two-winged Flies 351 



species of the genus is the notorious "frit-fly," one of the chief grain- 

 pests of Europe. 



SUBORDER PUPIPARA. 



Bird-collectors occasionally find on their specimens curious flat-bodied 

 insects with leathery Skin and a single pair of wings, which are obviously 

 parasites on the body of the birds. Owls and swallows seem especially 

 infested. Similar parasitic insects, but wingless, are also found on sheep, and 

 a winged form is not uncommon on horses. These degraded insects are 

 flies of the suborder Pupipara which are commonly known as -bird-ticks, 

 sheep- and horse-ticks, etc. The animals more rightly entitled to the name 

 "ticks" are really not true insects, but belong with the scorpions, spiders, 

 and mites in the class Arachnida. They have four pairs of legs and are always 

 wingless. Such true ticks are the leathery-skinned cattle-ticks, dog-ticks, 

 and wood-ticks. 



The degraded Diptera belonging to the suborder Pupipara, and also 

 called ticks, have of course three pairs of legs and some are winged. Their 

 name Pupipara comes from the curious circumstances of their birth. The 

 female does not deposit eggs outside her body, but gives birth to young which 

 are just ready to assume the pupal stage at the time of their appearance. 

 In the case of one species, the sheep-tick (Melophagus) , whose development 

 has been carefully studied, the female has four egg-tubes each of which 

 produces a single germ-cell at a time. Of these four egg-cells three remain 

 small, while one becomes large and develops into an embryo. This embryo 

 lies in the unpaired wide vagina of the female, soon casts off its egg-envelopes, 

 and is nourished as a growing larva by a secretion from two pairs of glands 

 opening into the vagina of the mother. Here the headless, footless larva 

 lies and grows until it is about inch long, when it is born and immediately 

 pupates. The development of the other Pupipara, as far as studied, is 

 similar to that of the sheep-tick. 



The suborder includes three families, as follows: 



With compound eyes; sometimes with wings. 



(Bird-, sheep-, and horse-ticks.) HIPPOBOSCID^:. 

 Without compound eyes, always wingless. 



Halteres present; on bats (Bat-ticks.) NYCTERIBIID^E. 



Halteres absent; on honey-bees (Bee-lice.) BRAULID^E. 



Of the Hipposcidae the sheep-tick, Melophagus ovinus, already referred 

 to, is common and familiarly known. It is wingless, and can crawl readily 

 about through the wool next to the skin. With its strong proboscis, com- 

 posed of two hard pointed flaps, it punctures the skin and sucks blood from 

 its host's body. The horse-tick, Hippobosca equina (Fig. 501), is winged. 

 There are several species of this family found on birds. Olfersia americana 



