88 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



As in the males, the epiphysis is of considerable help, since it is easily 

 visible, is usually relatively large, and curved similarly to the male's 

 (compare figs. 75 and 76), although rarely it may be reduced or absent. 

 In the vast majority of specimens which have been examined it is large, 

 however, while the epiphysis of females of the other species is either 

 absent or quite small. 



MATURE LARVAE (figs. 339-341).— Head light blue, motded with 

 black, sparsely covered with fine whitish-orange setae. Each abdominal 

 segment marked dorsally with a yellowish-buff, keyhole-shaped spot 

 which may be divided to form an anterior spot and a smaller posterior 

 spot, or the posterior spot may be absent. (All of these variations 

 can be found in populations of disstria from many parts of North 

 America.) Constriction of keyhole spot marked by a vertical black 

 dash or black spot on each side which includes setal group Dl. Imme- 

 diately below these marks at the dorsal edge of the subdorsal area is an 

 irregular, longitudinal reddish-brown line which is broken at the 

 intersegmental areas. Posterior to keyhole spot on each segment, three 

 short, irregular reddish-brown lines are usually visible, the outer one 

 sometimes connecting to the reddish-brown longitudinal line to form 

 a loop. Remainder of subdorsal area varying in color from bluish to 

 nearly black. If black, the vertical black dashes or black spots may be 

 obscured. Ventral edge of subdorsal area with a small black spot 

 surrounding setal group SD on each segment. Immediately below this 

 black spot runs the irregular, yellowish subdorsal line which is often 

 bordered dorsally with black. Supraspiracular and subspiracular areas 

 blue-gray to dark gray. Ventral area blue-gray to dark gray, usually 

 with a median black spot on each segment, and often with a dark gray 

 area running the full length of the body between the bases of the legs. 

 Dorsal setae black or orange; lateral setae whitish or orange. Setae 

 sparse compared with other species. 



LARVAL DIAGNOSIS. — Larvae oi disstria always can be identified 

 by the yellow-buflf, distinctively shaped spots on each segment. No other 

 North American larva is marked like this. In addition, the following 

 setal counts are useful in identification, especially early instars. On 

 abdominal segments 3-6 setal group Dl usually contains 4 setae, 

 and setal group LI usually contains 2 setae. Other North American 

 species usually have 5 setae in Dl and 3 setae in LI. The number of 

 setae in these groups should be checked on both sides of several seg- 

 ments to obtain a reliable count, since the number of setae in individual 

 groups may vary on some specimens. A Palearctic species, M. neus- 

 trium, is the only other species that has been examined which has 4 

 setae in Dl and 2 setae in LI. See the discussion of the Palearctic 

 species (page 187). 



