THE MELON FLY IN HAWAII. 



35 



are easily distinguished from each other by differences in structure 

 as well as in size. Unless otherwise noted the Newport or hidden 

 segment is not counted in numbering the 



segments. 



First larval instar (fig. 3). — Length about 2 

 mm. Mouth hooks small, about 0.45 mm. long 

 and shaped as illustrated in figure 4, a. Anterior 

 spiracles found in succeeding instars are lack- 

 ing. Posterior spiracles minute, being about 

 0.046 mm. across at the widest point. Stigmal 

 plates approximate, each with two instead of 

 three slits, as shown in figure ,5. The fine bristles 

 arranged in three groups about the stigmal plates 

 are somewhat longer than the width of the 

 tracheal trunks at their termination. 



Second larval instar 



Fig. 5. — The melon fly : Pos- 

 terior spiracles of first- in- 

 star larva. Enlarged. 

 (Original.) 



Fig. 6. — The melon fly : 

 a, Anterior spiracle of 

 third-instar larva ; b, an- 

 terior spiracle of second- 

 instar larva. Enlarged. 

 (Original.) 



Length about 5.1 mm. 

 The second-instar larva is sufficiently large to be 

 readily distinguished by its size from the first- 

 instar larva but not from undersized third-instar 

 larvae. It differs principally from the third-instar 

 larva by the shape of the anterior spiracles (fig. 

 6, b), which are terminated in about 18 to 20 

 lobes arranged in two groups as the result of a 

 depression in the center of the distal margin of 

 the spiracle. Spiracles about 0.10 mm. wide at 

 the widest point. The posterior spiracles (fig. 7) 

 less heavily chitinized than in the third instar ; 

 stigmal plates approximate, each with three slits 

 which are proportionately shorter than those of 

 the third instar. The bristles about the margins 

 of the stigmal plates arranged in four sets in- 

 stead of three as in the first instar and rela- 

 tively shorter and finer. Mouth hooks (fig. 4, &) 

 about 0.47 mm. long. 



Third larval instar (fig. 8). — Length about 10 

 mm. Head slightly bilobed, at least with two 



prominent thick fleshy protuberances. Each pro- 

 tuberance bears two pairs of microscopic papillae, 

 both on the distal portion, but one, the more dor- 

 sal, more antennalike in its formation, though 

 without a terminal style or bristle; the more 

 ventral pair but little elevated from the surround- 

 ing portions of the body, yet made quite evident 

 by several concentric rings. Mandibles or mouth 

 hooks very distinct, black, about 0.84 mm. long, 

 as illustrated in figure 4, c. Mandibles protrude 

 from two openings, one on each side and just an- 

 terior to the mouth opening. Between and just 

 anterior to the mandibular openings is a slight 

 protuberance. La brum distinct, more or less circu- 

 lar, though appearing variously in different alco- 

 holic specimens ; lobes about orifice about 20 in number. Anterior spiracles on 

 lateral margins of second segment fan shaped, with distal margin straight or 



Fig. 7. — The melon fly : Poste- 

 rior spiracles of second-instar 

 larva. Enlarged. (Orig- 

 inal.) 



