36 SUMMARY OF CURRENT RESEARCHES RELATING TO 



development of the valvular arrangements. Where proi^er valves are 

 developed it is obvious that we have a higher differentiation, inasmuch 

 as there is a wider division of labour. 



Paltosoma torrentium, a Fly with Dimorphous Female.*— This 

 species,! already distinguished by its very remarkable larva, has been 

 found by Dr. Fritz Miiller to possess two forms of the female, 

 differing so widely in the size of the eyes and in the structure of the 

 mouth-parts and feet as to lead to the inference that they have 

 entirely different habits of life. The case is a different one from that 

 of dimorphous females of Lepidoptera, for here neither female can be 

 said to resemble the male more closely than the other. The imagines 

 have not yet been reared from the pupae, as the latter die when 

 removed from their native streams, but the fully developed insects 

 have been removed from pupse which were ready to burst, and the 

 identity in species of the male and the two females has thus been 

 determined. 



The eyes serve readily to distinguish the three forms : in the male 

 they cover nearly the whole head and meet at its apex, excluding the 

 accessory eyes, which are placed on a special posterior pedicel ; in 

 one form of the female they include the whole length of the head, 

 but leave a broad space between them on the vertex ; in the other 

 females they are scarcely half so long or broad. 



The mouth-parts are fully j)resent in the large-eyed female alone ; 

 the labrum is pointed, and has the shape of a somewhat broad dagger ; 

 its edges are smooth, and look right and left ; beneath it lies another 

 style, narrower, toothed at the edges, and traversed by a canal, and 

 representing the poison-organs of the blood-sucking Dii)tera ; to the 

 right and left of this lie the long, narrow, and thin mandibles, armed 

 with teeth on their inner edges. The first pair of maxillae are weak 

 and bristle-like ; the fused second pair acts as a sheath enclosing all 

 the mouth-parts, and has a pair of palp-like organs on its upper 

 side. The essentially blood-sucking characters of these parts are wholly 

 wanting in the other — the small-eyed — female, and in the male. In 

 them the maxillae are entirely absent, the median style which lies 

 behind the labrum has no teeth, and its canal opens on its upper, not 

 under, surface. The main difference between the male and the small- 

 eyed female is that the apex of the labrum is hirsute in the one, naked 

 in the other. 



Differences similar to these which exist between the one female on 

 the one hand, and the male and other female on the other hand, are 

 known to occur between the sexes of those Diptera whose females 

 suck the blood of Mammals, and in several cases the honey-sucking 

 habits of the males have been observed. 



The differences between the feet in the three forms consist in the 

 slenderness and length of the terminal joint, which has a simple and 

 relatively small (y\ of its length) claw, in the small-eyed female ; in 

 the large-eyed female the claw is f of the length of the last joint, 

 and beset with hairs ; in the male the length of the claw is intermediate 



* ' Kosmos,' vii. (1880) p. .S7. t See this Journal, ante, p. 616. 



