FLIES 219 



concerned. The female requires a blood meal before she can lay fertile 



eggs. 



The eggs, which may number over 300, are deposited in jelly-like 

 masses on the edge of streams or scattered over the water. In some 

 cases the female skims above the surface laying an egg every time she 

 dips her abdomen into the stream, and again at other times she crawls 

 below the water to deposit her eggs on submerged vegetation and under 

 stones. 



The larva, which moults six times, has a fan-like structure round its 

 mouth with which it sweeps minute organic particles down its throat. 

 In order to be able to withstand a strong current it is provided with a 

 posterior circlet of spines by which it can anchor itself in the upright 

 position to stones and plants. In some streams there are very large 

 numbers of these larvae. A count once revealed 734 to a square inch on 

 a submerged branch. When the upper reaches of a stream begin to dry 

 up, which often occurs in the case of swiftly running rills or rivulets, the 

 larvae of some species migrate downstream. 



The pupa is enclosed in a sHpper-shaped silken cocoon spun by the 

 larva. When the fly is ready to emerge it uses a sort of Davis-escape 

 device. Air collects within the pupal skin until it finally bursts. The fly 

 is then carried to the surface in a bubble of air, without even getting its 

 feet wet — and darts away into the sunshine. Adult flies migrate many 

 miles from their breeding haunts, possibly helped by the wind. 



Birds destroy large numbers of black-fly. Chickens for example eat 

 them greedily, and when they approach a barnyard fowl singly it is 

 always a toss-up which will feed on the other. Aquatic birds also gorge 

 on the larvae which they skim off submerged vegetation and stones. 



Mosquitoes are carriers of various species of Filaria — nematodes 

 which complete part of their development in the insect. Black-fly are 

 also carriers of a related group of worms. Onchocerca. The larvae of these 

 worms are confined to the connective tissues just under the skin of the 

 infested mammal. They are consequently taken up by Simuliidae, 

 which do not drill straight into the blood stream like mosquitoes but 

 rasp a hole in the skin of the host. It has been claimed that the saliva 

 of the black-fly attracts the worms. 



Apart from Protozoa and nematodes there are several other para- 

 sites of these insects, but few, if any, are yet recorded from this country. 



Although Simulium venustum is the best known of the bird black-flies 

 in Britain, there are at least two other species, .S*. latipes and S. aureum, 



