481 



ON THE ECxGS AND VI POSITION OF PSOKOPHORA {JANTIIINOSOMA) 

 POSTICATA, WIED. (CULICIDAE). 

 By J. L. Pawan, M.B., Ch.B., 



Trinidad, British West Indies. 

 (Plate XVII.) 



The mosquito, Psorophora posticata, Wied. {Janthinosoma musica, Say), commonly 

 deposits its eggs in the rain-water that accumulates in the l:)roken cacao pods strewn 

 in heaps about the cool shady parts of cacao fields. 



The ovipositing female assumes a characteristic attitude upon the surface of 

 the water. The hind pair of legs lie extended backwards to their maximum length 

 and slightly outwards. The front pair of legs are projected forwards and outwards, 

 the femur forming almost a right angle with the tibia and the latter an obtuse angle 

 with the tarsus, which rests forwards and outwards. In both the front and hind legs 

 the tarsi are the only parts in direct contact with the surface of the water. The 

 position of the middle pair of legs is very definite ; the femur is directed backwards, 

 the tibia is acutely flexed forwards and its apex embraces firmly the lateral borders 

 of the egg-mass, the tarsus projecting sharply backwards and resting upon the sruface 

 of the water. Whilst the coherency of the egg-mass is maintained by the grip of 

 the middle tibiae, the buoyancy of the eggs prevents the mosquito from being 

 submerged during the process of oviposition. This process, which usually takes 

 place in the morning, lasts from two to four hours, during which time the insect 

 seems quite helpless, being unable and unwiUing to take wing if disturbed, rendering 

 herself an easy captive, and giving one the impression of being in pain. After 

 oviposition has ceased the mosquito continues to rest upon the surface of the water, 

 often away from the egg-mass, for from two to three hours, then crawls to the side 

 of the containing receptacle and flies away. 



The eggs lie in circular or subquadrate masses, consisting of from 25 to 40 in 

 number, floating with their long axes perpendicular to the surface of the water 

 and glued together at their broadest circumference by a gelatinous substance that 

 helps to keep the mass afloat. A little less than one-third of their length is sub- 

 merged. The eggs on escaping from the female are of a distinct greyish-brown 

 colour, but in less than half an hour that portion which is exposed to the air 

 assumes a dark steel-blue appearance, the whole mass of individual eggs simulating a 

 honeycomb. 



To the naked eye an individual egg (PI. xvii, fig. 1) shows an ovoid shape elongated 

 at both extremities, with the greatest circumference immediately above the junction 

 of the blue and brownish portions, the latter tapering rapidly with a sharp and 

 distinct curve. The lower portion, which retains its colour, is chitinous, while that 

 which lies above the water is brittle and calcareous. 



Under the microscope the portion of the egg that rests above the sin-face of the 

 water is seen to be covered with a definite but loosely adherent capsule, studded 

 with numerous translucent pedunculated and sessile tubercles, arranged in regular 

 rows and containing air (PI. xvii, lig. 2). This investing capsule does not extend 

 beneath the water, and is also absent from a narrow triangular area on the upper two- 

 thirds of the egg. Along this bare area the brown egg-shell can be seen, for the 

 change in colour from brownish to dark blue affects the capsule only and not the 

 egg-shell. Shorn of its capsule the underlying egg-shell is seen to consist of a brownish 

 outer layer made up of definite circular strands, enclosing an inner thin delicate 

 la^-er surrounding the yolk substance, which bathes the nucleus and subsequently 

 the embryo. No operculum can be seen. 



The larva hangs with its head downwards in the floating eg^, and in from 

 eight to ten hours after oviposition ruptures the lower submerged portion and 

 the longitudinal area of the e^g free from investing capsule and then escapes into 

 the water. 



