2i8 INSECT TRANSFORMATION 



increase largely the risk of infection and the numbers of the 

 parasites. The sheep harbours the larvae of one of these 

 specialized parasitic flies (Oestrus ovis), a greyish, heavily- 

 built insect with vestigial mouth-parts so that it takes no food 

 in the adult state. The female Oestrus lays eggs, or deposits 

 active maggots which have hatched within her body, in the 

 sheep's nostrils, whence the maggots make their way into the 

 nasal air passages, and subsequently into the frontal sinuses 

 of the head where they become fully grown. Being finally 

 coughed out by the sheep, they pupate on the ground in 

 preparation for the flies of the next generation. 



An excellent example of specialized parasitism is afforded by 

 the Ox Warble-flies (Hypoderma) , usually grouped in the same 

 family 1 as Oestrus, which they resemble in their reduced jaws 

 and abstinence from feeding in the winged condition. They 

 are hairy flies, coloured like bumble-bees, and flying in the 

 summer sunshine with a distinct hum. The " fly season ' 

 for our two native species of Hypoderma (H. bovis and H. 

 lineatum} lasts from May till September. The female fly 

 (Fig. 109 a) pursues cattle grazing in the fields, and lays her 

 curiously-shaped eggs (Fig. 109 c), each with a grooved base, 

 astride the beast's hair usually low down on the legs. The 

 larva of Hypoderma is a muscoid maggot, modified for its 

 special mode of life, and passing through somewhat marked 

 changes of form in the course of its life-history in correlation 

 with its migrations through the host's body. The first-stage 

 maggot (less than a millimetre long) is elongate and very 

 spiny, with powerful divergent mouth-hooks between which 

 is a strong median spine formed of paired sclerites (Fig. 109 /). 

 This formidable armature is used by the maggot first for 

 breaking its way out of the egg-shell, and then, for boring 

 into the skin of its host ; 2 the little larva crawls along the 

 hair and uses the hair-follicle as a starting-point for its tunnel- 

 ling operations. 3 Working along beneath the skin, the maggots 

 migrate to the gullet of the host, for they may be found in 



1 Bracy Clark : " An Essay on the Bots of Horses and other Animals ". 

 London, 1815. F. Brauer : " Monographic der Oestriden ". Wien, 1863. 



2 G. H. Carpenter and T.R. Hewitt : " Some new Observations on the 

 Life-history of Warble-flies ". Irish Nat., XXIII. 1914. And Jonrn. 

 Dept. Agric., Ireland, XV. 1914. 



3 S. Hadwen : " Contribution to the Biology of Hypoderma lineatum ". 

 Bull 21 Health of Animals Branch, Canad. Dept. Agric. 1916. 



