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381 
cussed, and comparisons are made with the natural history of other 
mycetophilids. 
Appendix 
Details of larval spinning behavior 
The spinning behavior of Leptomorphus subcaeruleus larvae was 
complex, and it is likely that an evolutionary series of behavior may 
eventually be established between this species and others whose 
spinning behavior is less elaborate. The following description, derived 
from study of 24f/sec. movies of two L. subcaeruleus larvae laying 
trails on the undersurface of clean saranwrap, will hopefully serve 
as a basis from which to start such a study. 
Silk laying involved three steps repeated over and over: the larva 
swept slowly once or twice in an often nearly semicircular arc in 
front of itself with its outstretched head, laid silk by swinging its 
head quickly from side to side, placed its anterior end against the 
silk and moved forward, and swept slowly again. Larvae usually 
tapped their heads on the surface on which they moved several 
times during one of the slow sweeps, but occasionally a larva did 
not touch the surface at all. No silk was laid during sweeping 
activity since there were no lines running in the direction of the 
semicircular sweeps on glass slides on which larvae had moved. 
Sweeping motions were much slower than swinging movements: one 
larva averaged about 0.7 seconds for sweeps of about 120°, but took 
only 0.04-.08 seconds to swing approximately the same distance while 
spinning silk. Larvae usually modified sweeping motions (often by 
repeating the sweep) when they encountered an object such as an- 
other larva’s body during a sweep. It appears that sweeping is 
a type of exploratory behavior, probably functioning to guide the 
animal as it moves. 
Direct observations of active larvae and examinations of glass 
slides on which they had moved showed that silk was spun during 
the swinging movements following sweeps. Eight or (often) fewer 
equally spaced threads of equal diameters were laid down with each 
pass of the larva’s head, indicating that silk from the spigots on the 
labrum was laid during swinging. Additional threads with smaller 
diameters were also laid. The threads were attached to the slide 
near the edges of the silk trail, and the attachments appeared to 
consist of unpolymerized thread material: each line, whose diameter 
was uniform between attachments, spread out into a long “puddle” 
at each attachment. 
