82 



as partly submerged trees, etc. Eggs are deposited at or below the 

 surface on smooth rocks, posts, or debris. The second group, con- 

 taining *S. vittatum, S. venustum, S. johannseni and S. bracteimi, is- 

 found in small streams with less current and with submerged herbage 

 on which eggs are deposited. The time of day at which oviposition 

 occurs is from 4.30 p.m. until 5.30 p.m. or until dusk. The most 

 suitable weather conditions are sunshine and lack of wind, while the 

 maximum temperature at which egg-laying can take place is 95° F. 

 The earliest record of oviposition at Dallas, Texas, was on 25th March, 

 and the latest at Spartanburg, S. C, on 2nd November. About 500 

 eggs may be laid by a single female at one time. Experiments at 

 Spartanburg have shown that eggs and young embryos are unable to 

 hatch after undergoing desiccation. Outbreaks of Simulium in large 

 rivers in early spring are due to the fact that desiccation is prevented 

 by the rising of the water at this period. The eggs hatch either in 

 still or running water, the period of incubation in the different species 

 varying from 7 to 12 days. The young larvae require swiftly flowing 

 water for their development. The sudden appearance of Simvlium 

 in new localities may be due to a change in distribution occasioned by 

 an increase in the swiftness of the current by heavy rains. The length 

 of the larval stage in summer is about 17 days. The food is entirely 

 microscopic, the larvae apparently thriving best in streams containing 

 Euglena viridis and Spirogyra. The streams in South Carolina which 

 are contaminated by chemical refuse from cotton mills are entirely 

 free from larvae ; this fact is of importance in carrying out control 

 measures. Pure animal sewage is not deleterious to growth, provided 

 other factors are favourable. The duration of the pupal stage at 

 Mumford, N.Y., is three weeks ; at Havana, 111., the maximum period 

 in late autumn is nine days at a temperature of 36° F. The minimum 

 period in the case of S. venustum at Spartanburg during June is 84 

 hours, at a temperature of from 70° to 90° F. The emergence of the 

 adult is hastened up to 90° F. In the Southern States breeding is 

 continuous from the middle of March until the end of November, 

 the life-cycle occupying approximately four weeks. In South Carolina 

 there are five or six generations annually, except in the case of S. pictipes, 

 which normally has three generations. In IlHnois, S. venustum has- 

 three or four generations each year. 



The larvae are often parasitised by Nematode worms of the genus 

 Mermis and by Myxosporidia. Hydropsyche sp. and Cyprinid fish 

 prey upon this stage. The adults are parasitised by Mermis sp. and 

 by the trypanosome, Crithidia simuliae, and are attacked by a wasp, 

 probably Monedula signata. The supposition that species of Simulium- 

 are capable of transmitting virulent charbon, cholera among chickens 

 and pigs, and pellagra, has been put forward by various observers. 

 In an attempt to determine whether such a relation exists, investi- 

 gations on the biting and feeding habits of S. venustum were carried 

 out. The results were as follows :■ — (1) In all adults taken while 

 ovipositing, apparently digested blood was present in the stomach ; 

 (2) no eggs within the ovaries developed without engorgement and the 

 requisite time to digest the blood meal ; (3) adults apparently fed 

 again after oviposition ; (4) the males had reduced mouth-parts and 

 were not engorged with blood, indicating the acquisition of this habit 

 by the females for a special purpose. 



A very complete bibhography is given. 



