1983] 
Root & Messina — Exema canadensis 
71 
Table I: Head capsule widths and case lengths (in mm) ol the immature stages of 
E. canailensis. 
Egg 
' 
11 
111 
IV 
Head width 
x(SE) 
0.29(±.01) 
0.37(±.02) 
0.50(±.02) 
0.67(±.02) 
N 
7 
7 
7 
9 
Case length' 
x(SE) 
1.05(±.05) 
1.62(±.38) 
2.69(±.33) 
3.55(±.32) 
4.34(±.26) 
Range 
0.96 1.12 
1.04 2.32 
2.16 3.36 
2.80 4.08 
3.84 4.80 
N 
12 
23 
18 
32 
45 
'As measured from case opening to tip of original egg case. 
substrate and the anterior end faces the nipple at the tail. Fully 
sclerotized adults cut a circular cap in the tail of the case with their 
mandibles; this cap is pushed off as the beetles emerge. The barrel- 
shaped, tuberculate adults (Figs. 9 10) can be easily mistaken for 
caterpillar frass by humans (Jenks 1940; Karren 1964; and our per- 
sonal experiences). It may be that vertebrate predators overlook 
them in the same way. The adults exhibit the widespread chryso- 
melid trait of quickly withdrawing the legs and dropping off the 
substrate when they are disturbed. This escape mechanism is elabo- 
rated in Exenia\ the deep sternal grooves (Fig. 10) allow the adult to 
retract its appendages so completely that the falling beetle bounces 
and rolls off the foliage. The compact adults also slide deeply into 
the litter beneath the plant and often come to rest in a deep recess 
where they are extremely difficult to find. 
Natural enemies 
No predators were seen to attack the larvae of E. canadensis 
during the many hours that we and our associates, E. W. Evans and 
J. A. Gowan, have spent observing the goldenrod fauna in the field. 
The three species of predaceous insects used in our experiments, 
however, were frequently observed to kill the larvae of other chry- 
somelid species that are associated with Solidago in central New 
York (Evans 1982; Messina 1982). In the laboratory, exposed E, 
canadensis larvae were readily captured and eaten by these preda- 
tors (Table 2). In contrast, few encased larvae were consumed even 
though the confined space in the petri dishes must have increased 
