GNATS 157 



mosquitoes — the name is indifferently used, and has no 

 scientific appUcation — are amongst the most graceful 

 and most beautiful insects that we know, but they have 

 been judged by their works, and undoubtedly are 

 unpopular, and we shall see that this unpopularity 

 is well deserved. Gnats belong both to the genus 

 Culex and to the genus Anopheles. The genus Culex, 

 from which the order takes its name, includes not only 

 our commonest gnat, often seen in swarms on summer 

 evenings, but some hundred and thirty other species. 

 Members of this genus convey from man to man the 

 Filaria noctuma, one of the causes of the widely-spread 

 disease filariasis, one variety of which is the elephan- 

 tiasis, so common in parts of the tropics. In patients 

 suffering from this disease minute embryonic round- 

 worms swarm in the bloodvessels of the skin during the 

 hours of darkness. Between six and seven in the even- 

 ing they begin to appear in the superficial bloodvessels, 

 and they increase in number till midnight, when they 

 may occur in such numbers that five or six hundred may 

 be counted in a single drop of blood. After midnight 

 the swarms begin to lessen, and by breakfast-time, 

 about eight or nine in the morning, except for a few 

 strayed revellers, they have disappeared from the 

 superficial circulation, and are hidden away in the 

 larger bloodvessels and in the lungs. 



In spite of their incredible number — some authorities 

 place it at thirty to forty millions in one man — these 

 minute larval organisms, shaped something like a 

 needle pointed at each end, seem to cause little harm. 

 It might be thought that they would traverse the walls 

 of the bloodvessels and cause trouble in the surround- 

 ing tissues ; but this is prevented by a curious device. 

 It is well known that, like insects, round-worms from 

 time to time cast their skins, and the young larvae in 

 the blood cast theirs, but do not escape from the 

 inside of this winding-sheet ; and thus, though they 



