14 STRUCTUEE OF CERTAIN DIPTEROUS LARV^. 



above it is the pair of mandibles or great hooks (fig. 1,1)), sometimes 

 fused into one. At the base and to one side of the mouth orifice is a 

 striated semicircular flap or lobe, called the stomal disk. These are 

 more or less distinct according to the amount of distension and 

 varying with the species. Below the mouth is a short transverse 

 piece, the labium. Behind the head segment there is another seg- 

 ment which is only clearly seen when the head is greatly protruded; 

 this hidden segment is known as ''Newport's segment," and the writer 

 has not counted it in treating of the number of segments, since it was 

 not considered by Portchinski and others in describing these larvae. 

 The second (apparent) segment bears on each side, in the full-grown 

 larvae, a short fan-shaped process, the anterior spiracle (fig. 1, c); 

 each spiracle shows at its tip a number of lobes, varying, with the 

 species, from four or five to forty or more. Some of the segments 

 show fusiform areas on the venter and on the sides; those on the 

 venter are called ventral fusiform areas (fig. 1, d) and those on the 

 sides, lateral fusiform areas (fig. 1, e). The last segment of the body 

 has a large area (fig. 1, i^) on the end containing two chitinized sub- 

 circular plates, the posterior stigmata. These, in the adult, have 



Fig. 1.— Larva of a muscid: a, Papilla of head; 6, great hooks; c, anterior spnacles; 

 d, ventral fusiform area; e, lateral fusiform area; /, aual tubercle; (], apical spines; 

 h, stigmal field (containing posterior stigmal plates). Enlarged. (Original.) 



each three slits or sinuous apertures and sometimes a rounded mark, 

 or button, at their base. In some forms the area or field around the 

 stigmal plates is sunken, forming a deep cavity, with the plates at the 

 bottom. The margin or lips of the cavity often bear conical proc- 

 esses (fig. 1, g), normally 12 in number. At the lower base of the 

 last segment is the anal field. Each side of the anus is a smooth, 

 convex area or lobe, whose size depends upon the extent of protrusion, 

 and above is a transverse swollen area, often spinulate, and some- 

 times with a cone at each corner (fig. 1,/). 



Within the head or, rather, anterior part of the body is a chitinous 

 framework, consisting of several articulated parts, called the cephalc- 

 pharyngeal skeleton. The outer part is the hypostomal sclerite, to 

 which are articulated the mandibles or great hooks; behind the 

 hypostomals is a pair of large sclerites, nearly divided to their base; 

 one branch is very thin and broader and longer than the other, which 

 is more chitinized, and rounded at tip; they have been called the 



