Apr. 5,1924 
Anatomy of the Apple Maggot 
5 
The posterior larval spiracles (PI. 1, A, PSp) are present in each of the larval 
instars. In the first instar each spiracle has two apertures (Pl. 2, D), in the 
second (PI. 2, B) and third (PI. 2, C) three apertures. The openings are sur¬ 
rounded by dark chitinous rims known as the stigmatic plates (PI. 2, D, SpPl ), 
which have different shapes characteristic of each instar. Beneath the plates are 
stigmatic chambers ( SpC ) set on the ends of the tracheae, which are similar in 
structure to those of the anterior spiracles and have the same yellow color. 
The stigmatic plates of the posterior spiracles of the first-instar larva are 
simple oval rims about each aperture, widest on the outer sides (PI. 2, D, SpPl). 
Four groups of delicate radiating hairs arise from the cuticle at the upper, outer, 
and lower margins of each pair of plates. The stigmatic chambers (SpC) are long, 
tubular end-sacs of the tracheae ( Tra ), in line with the tracheal axes. 
In the second instar the posterior stigmatic plates are elongate-oval in form 
and each has from six to eight teeth on the inner edge of each lip guarding the 
aperture to the stigmatic chamber (PI. 2, B). Four large, fan-shaped bunches of 
hairs arise from the cuticle around each spiracle, one above, one below, and two 
lateral between the outer ends of the plates. Each stigmatic chamber (SpC) 
divides distally into three lobes corresponding with the three stigmatic plates and 
their apertures. On the inner side of its base is a small circular scar connected with 
the body wall by a shrunken strand of cuticle ( v ). This, as in the case of the 
anterior spiracles, is the remnant of the channel through which the first stigmatic 
chamber and the lining of the trachese were drawn out at the first molt. The 
position of the new spiracles, however, is reversed at the two ends of the body; 
in front the new spiracle being formed mesad to the old. 
In the third instar the apertures of the posterior spiracles are linear slits in 
long, slender stigmatic plates (PI. 2, C). Each plate is a cylindrical chitinous 
structure at the end of a wide lobe of the stigmatic chamber, and is open to the 
exterior the full length of its outer surface and at each end (PI. 2, E). The lips 
of the opening are fringed with many slender prongs that curve inward like the 
teeth of a pair of rakes turned toward each other. The structure of these spira¬ 
cles is particularly conspicuous when a larva is submerged in water, for the air 
in the spiracles may then be seen glistening like mercury between the prongs of 
the slits of each plate and bulging out at the unguarded ends. Four radiating 
groups of hairs arise from the cuticle about the stigmatic plates, as in the second 
instar, but in this, the th^rd instar, most of the hairs are branched. The stigma¬ 
tic chambers (PI. 2, C, SpC) are much larger than in either of the other instars, 
and the scars in their bases, left by the chambers of the second instar, are large 
and conspicuous. These spiracles do not reappear on the pupa. When the 
stigmatic chambers and tracheal linings are cast out at the last larval molt 
the holes in the body of the pupa close. 
THE TRACHEAL SYSTEM 
The most conspicuous features of the larval tracheal system are the two large 
dorsal trunks that extend between the anterior and the posterior spiracles (PI. 
2, G. Tra). The trunks are connected by a dorsal commissure in each segment 
but the first, and they give off 11 descending lateral branches. The first and the 
last dorsal commissures (q and s) are much larger than the others and go straight 
across from one longitudinal trunk to the other. The rest (r) are very slender 
tubes and each makes a long V-shaped loop into the preceding segment. The 
lateral branches from the dorsal trunks (PI. 2, F) supply tracheae to the viscera 
and to the body walls. All but the first two are connected below the middle of 
the sides by a series of longitudinal commissures (t). These commissures are 
seldom shown in drawings of the tracheae of the maggot, but they are a very 
important feature of the general tracheal system; they constitute, in fact, the 
