A Fly in Ant's Clothing 



Beware of larval imposters 

 by Gregory Paulson and Roger D. Akre 



As the sun rises, the dull thud of an ax 

 echoes through a valley in northern Idaho. 

 A group of elk, startled by the sound, 

 begin to move purposefully toward the 

 forest, when the sudden roar of a chain 

 saw sends them headlong toward the shel- 

 ter of the trees. The cause of this commo- 

 tion is not another logging operation; 

 rather, it is our research team in search of 

 Microdon — the subject of our long-term 

 study. Microdon are syrphid flies, also 

 known as flower or hover flies, and they 

 live most of their lives in the nests of so- 

 cial insects. Although some tropical Mi- 

 crodon live with wasps, the North Ameri- 

 can species we study are associated only 

 with ants. We have gathered them from 

 colonies of carpenter ants and from the 

 nests of Formica ants in stumps and logs. 

 Hence the need for our sophisticated col- 

 lection equipment: an ax, a pry bar, and a 

 chain saw. 



Most of our coUecting expeditions have 

 been carried out in the northwestern 

 United States, particularly in northern 

 Idaho. We have also "stalked" Microdon 

 in the Midwest, from the Black Hills of 

 South Dakota to the forests of northern 

 Minnesota. While Microdon are fairly 

 easy to find once you know exactly where 

 to look, tiiey are not common. This prob- 

 ably holds true for most inquiUnes, insects 

 that reside in the nests of other insects. 

 Most have a parasitic or predatory symbi- 

 otic relationship with their hosts. Their 

 strategy is to live in the midst of their 

 hosts — and subtly live off them — without 

 being detected. 



Each year we begin our studies as soon 

 as the snow melts. Mature Microdon lar- 

 vae overwinter deep within the ant nest. In 

 spring, they move to the surface of the nest 

 to pupate. This is when they are easiest to 

 find and extract. If the ant colony is within 

 a decayed stump or log, as is frequently 

 the case, the larvae and pupae wiU be read- 

 ily visible when the wood is split open 

 with an ax or pry bar. Microdon, like all 

 "higher" flies, pupate within a chamber, or 

 puparium, formed from the skin of the 

 final larval stage. The larvae secrete a glue 

 that tightly bonds with the wood and holds 



the puparium in place as the pupa devel- 

 ops and, later, as the adult emerges. Adult 

 Microdon are quite hairy and range from 

 gray to orange depending on the species. 

 Microdon piperi adults are a striking 

 metallic green and are strong and agile 

 fliers. They live only long enough to mate 

 and lay eggs, often in the same nest from 

 which they fliemselves emerged. We are 

 most intrigued, however, not by the beau- 

 tiful adults, but by the biology, morphol- 

 ogy, and behavior of the immamre forms 

 of Microdon. 



Perhaps because of their sluglike ap- 

 pearance, Microdon larvae were at first 

 misidentified as mollusks and later as scale 

 insects; their true identity as flies was not 

 revealed until the 1880s. How they survive 

 was long debated, but since the 1970s, sci- 

 entists have known that some species prey 

 on ant larvae. The extent and the exact 

 mode of predation were unknown until 

 1985, when one of the larval sttategies 

 came to hght. 



In an experiment, Wilham Gamett, of 

 the University of Cincinnati, placed many 

 first instars (the first of three larval stages) 

 in a glass-sided observation nest complete 

 with host ants and their brood. Previously, 

 most entomologists had thought fliat the 

 first instars dispersed immediately upon 

 hatching, settling deep within the ant nest. 

 In this experiment, most of the larvae 

 under observation had disappeared and 

 were thought to be dead. One remained, 

 however, and at about 1/32 inch long was 

 visible only through a dissecting micro- 

 scope. It was clinging to the outer surface 

 of an ant cocoon. The magnification re- 

 vealed the larva becoming rounder and 

 rounder, as if it were exerting pressure to 

 distort its shape. Suddenly, it was simply 

 gone. A httle time and deductive reason- 

 ing led to the conclusion that the larva had 

 inserted its mouth hooks into the silken 

 cocoon and created a hole large enough to 

 allow it to enter. When the instar had ex- 

 erted enough pressure and the hole was 

 large enough, the larva quickly popped in- 

 side (and a new term, "pupa poppmg," 

 was coined). The disappearing larvae were 

 simply inside the cocoons, feeding on the 



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