THE CHEESE SKIPPER AS A PEST IN CURED MEATS 25 



and the posterior part of the sloughing-off linings of the tracheae and the 

 posterior spiracular structures. The same inpulled condition is typical of 

 second-stage skins. 



To the anterior extremity of first-stage skins a flagellum-like appendage 

 usually adheres. This probably consists of the gut lining, and appears to 

 inclose the cephalopharyngeal skeleton. The anterior end of the first-stage skin 

 also bears a pair of two-jointed, fleshy antennae and a pair of minute, in- 

 distinct areas bearing four or five dark spots which the writer assumes to be 

 structures functioning as eyes. 



The second-stage larval skin, about 4 millimeters in length, lacks the whip- 

 like appendage of the first-stage skin, but has a pair of prominent, slender 

 mouth hooks and a delicate cephalopharyngeal skeleton. The antennae and eye 

 spots are present in all stages. 



THIRD-STAGE LARVA 



The third-stage larva, when full-grown and ready to migrate from its food 

 in search of a pupation place, is about 6 to 8 millimeters long, the ultimate size 

 depending on food and environment. There are 12 segments, including the head. 

 This stage has a robust cephalopharyngeal framework and strong mouth hooks, 

 and is whiter and more opaque than the preceding stages. 



The tracheal system is clearly visible in specimens immersed in a liquid, the 

 confined air giving it the appearance of being filled with mercury. Both pairs 

 of spiracles are yellow ; the posterior ones are provided with conspicuous yellow, 

 bulbous bases or stigmatic chambers. The ends of the spiracles are very small 

 and the pattern of the openings (three oval apertures) is not easily seen. The 

 tracheal system of the full-grown larva, seen from the side, is shown in Figure 

 4. The two halves of the tracheal system are joined dorsally by commissures, 

 of which the first and last are of greatest diameter. 



Fig. 4. — Tracheal system of migrant larva of Piophila casei. Left-hand 

 half of system shown. The dorsal tracheal trunks are joined by com- 

 missures ; these are not visible from the side. X 10 



The modes of progression of the mature larva are two : Skipping, 

 and creeping along by peristaltic movements of the body wall, aided 

 by transverse rows of ambulatory teeth on the ventral surface of the 

 last seven intersegmental lines and at the anus. The microscopic 

 ambulatory teeth are triangular in shape, are directed posteriorly, 

 and are arranged in triple rows, except at the anal region where only 

 a few teeth are present. 



The skip frequently propels the maggot 10 inches horizontally or 

 6 inches vertically. As Swammerdam (73, pp. 63-6 Jf) tersely 

 expressed it, " the worm leaps with a surprising violence," and it is 

 "surprisingly strong, and has a most vigorous constitution." In 

 skipping, the insect bends its body in the shape of a ring and hooks 

 its oral claws over the sharp angle formed by the ventral edge of the 

 posterior beveled truncation. The larva then pulls hard, so hard that 

 the two halves of the body are brought together by the strain, and 

 the hold is suddenly released, the resulting snap throwing the insect 

 into the air. The act is comparable to the behavior of a strip of 

 whalebone when the two ends, which have been held together, are 

 released. The process.of adjusting the hooks is deliberate, the larva 

 using considerable care in placing them before the strain is brought 

 to bear. Although the skip is useful when the larva is alarmed and 

 6102°— 27 4 



