PRINCIPAL CACTI'S fXSECTS OF UNITED STATES. 



37 



SCAVENGERS. 



In the list of insects found associated with the cactus plant at the 

 end of this bulletin we have included T3 species in the category of 

 scavengers. Many of these species feed only upon the joints when 

 these have been killed by other insects or when they are blown to the 

 ground. A considerable number of the scavengers, however, breed 

 in the living joints, obtaining entrance through the mines of Moneil- 

 ema, Melitara, and other forms. The diseased condition caused 

 primarily by the original inhabitant of the joint is increased by the 

 work of such scavengers. They are therefore incidentally injurers of 

 the plant. The cavities they inhabit become infested by various 

 fungi and bacteria and the diseased area increases in size when, with- 

 out the intervention of these scavengers, the plant would be able to 

 heal the wound. 



Copestylum marginatum Say. 1 



The most common of the 

 scavengers which increase the 

 effects of the attack of other 

 insects is Copestylum margin- 

 atum Say (fig. 6). The adults 

 of this fly are to be found about 

 the cactus plant from March to 

 October. They are also taken 

 commonly in the blooms of a 

 long list of plants found in the 

 cactus region. Undoubtedly 

 they breed in decaying vegetation of all kinds, but one of the most 

 important breeding places is the joints of cactus that have been 

 injured by Melitara, Moneilema, Gerstseckeria, and other forms. 

 Very soon the interior of the joint becomes filled with a dark, mal- 

 odorous liquid, which undoubtedly causes the rapid decay of the 

 plant tissues. 



The adult fly deposits its eggs on the spines in large masses. (PL 

 VII. fig. 3.) 



The larva of this species measures 20 mm. by .4 mm.; the tail is 

 i mm. in length. It is shining, its skin wrinkled. In color it is white, 

 the tail dark brown. Each ventral segment has two almost contiguous 

 oval areas of very short, stout, brownish spines, and there are similar 

 spines on the head segment. The puparium is 10 mm. by 4 mm., 

 calcareous, its surface dirty whitish, covered with pa-rticles of sand. 

 There are many annulations of spinose areas, more distinct beneath. 



Fig. 6. — A cactus insect, Copestylum margin- 

 atum: Adult. Enlarged. (Original.) 



1 Order Dipt era, Family Syrphidse. 



