BLOODSUCKING INSECTS AND TROPICAL DISEASES. 165 



This family consists of the single genus Simulium, which is universally dis- 

 tributed, and of which some seventy species, difficult to distinguish one from 

 another, have been described up to the present time. The females of some of 

 these flies, which are among the most dreaded of all blood-sucking Diptera, 

 sometimes occur in enormous swarms, and by their attacks upon horses, mules 

 and cattle, especially in certain parts of the United States, occasion great losses 

 among these animals, besides frequently molesting human beings. In the dis- 

 trict of South Hungary called the Banat the Columbacz Midge (Simulium 

 columbaczense, Schonb.) has been notorious for more than a hundred years, 

 owing to the destruction caused by it among cattle. 



Among the foot-hills of Himalayas in North Lakhimpur, Assam, where the 

 flies are locally called " Dam Dims," the poisonous bites of Simulium iridicum, 

 Becker, are troublesome to tea-coolies, and in the Western Himalayas during 

 the hot season " Potu " flies are a well-known scourge ; ifc is stated that when 

 the Chakrata-Saharanpur road was under construction numbers of the coolies 

 employed on the work died from the effects of their bites. 



Appearance. — Small black or grayish flies, varying in length from 1£ to 4 mm. 

 according to the species, with a conspicuously humped thorax, short straight 

 antennae, delicate iridescent wings, stout legs, and with the proboscis not pro- 

 jecting. In the male the eyes appear to occupy the whole of the head, and 

 meet in the middle line above ; in the female they are smaller and separate. 



Life-history. — The preliminary stages are passed in running water. The 

 eggs are deposited in a compact layer or gelatinous mass on stones or plants 

 close to the water's edge. The larval stage lasts for about four weeks in the 

 summer in temperate climates, though longer in cold weather, and the winter 

 is passed in this state. According to Johannsen, the full-grown larva of even 

 the largest American species does not exceed 15 mm. in length. In shape the 

 larva is somewhat cylindrical, broadest posteriorly, where it is attached by 

 means of a sucker to a stone, the stem of a water-plant, a dead leaf, or other 

 object. The larva is able to shift its position by crawling in a looping fashion 

 but usually remains in a more or less erect attitude. It feeds on alga?, dia- 

 toms, and parts of phanerogamous plants, which are brought to the mouth by 

 means of the currents set up by two broad fan-like organs situated upon the 

 head. In colour the larva varies according to the species, and perhaps also to 

 some extent in accordance with its food, from deep shining black to yellow or 

 dark green. When mature, the larva spins a silken cocoon, within which 

 it pupates and in which the pupa remains motionless, breathing by means of a 

 pair of branched respiratory filaments which project from behind the head. 

 About a week is occupied in the pupal stage, and then the perfect insect, mak- 

 ing its escape through a rent in the back of the thorax, ascends to the surface 

 in a bubble of air, and makes its way to some support on which it rests until 

 its tissues are sufficiently hardened to enable it to fly. 



Habits of the perfect insects. — The males, which are incapable of sucking blood, 

 are fond of dancing in the sun in swarms at some height in the air ; the 



