400 THE BIOLOGY OF INSECTS 



extent by simple physiological reflexes, but with apparently 

 purposeful reference to the course of the life-history of the 

 insects. Some further details of the form and habits of 

 these flies and their maggots may now serve to illustrate 

 their adaptive relations with their hosts. The flies are 

 hairy insects with sufficient resemblance to bumble-bees 

 to suggest " mimicry," were it not that their habits are such 

 as to give little hint of utility in their likeness to stinging 

 Hymenoptera. They are on the wing from mid-May till 

 early September- — a succession of individuals, as the flies 

 with aborted mouth-parts cannot feed and must be short- 

 lived. The two species are largely seasonally isolated, 

 H. Imeatum appearing in May and June, H. hovis in July 

 and August ; thus together they have opportunity of egg- 

 laying all through the warm weather. They fly in sunshine 

 with a hum distinct though not loud, and at their approach 

 the cattle run about in apparent terror, with their tails 

 elevated — a remari^able and inexplicable fact as the flies 

 can neither sting nor bite. Because of this effect of the 

 flies on the cattle, the name *' gadfly " has been applied to 

 them as to the large Tabanidae already mentioned in this 

 chapter. When a female warble-fly " strikes " at a beast 

 the telescoped tail-segments are partly extruded and the 

 eggs, held by the processes of the ovipositor (Fig. 35, C), 

 are laid on the hairs, each egg being provided with a short 

 stalk expanding into a grooved elliptical base which fits 

 neatly astride the hair, and is secured in place by a maternal 

 secretion which hardens to form a cement. It is interesting 

 to note an apparently constant distinction between the two 

 species of Hypoderma in their egg-laying habits : the larger 

 eggs of H. hovis are placed singly close to the base of a hair, 

 while the smaller eggs of H. lineatum are arranged in rows 

 of seven to twenty along the middle region of a hair, their 

 grooved bases in close contact (Plate II). Perhaps because 

 the former species gets nearer to the beast's skin its attentions 

 seem usually to irritate the cattle more than those of the 

 latter, as S. Hadwen (1912, 1919) has observed. About four 

 days after laying, the eggs are hatched, the larva being by 



