1983] 
Root & Messina — Exenia canadensis 
11 
rates. This low output is reflected in the females’ reproductive mor- 
phology. In dissections we found that females of E. canadensis have 
only four or five ovarioles per ovary and that each ovary never 
contains more than one fully mature oocyte. Camptosomate beetles, 
in general, have relatively few ovarioles per ovary (Robertson 1961 ; 
Suzuki 1974; Mann and Singh 1979). Beetles may compensate for 
gradual egg production by ovipositing over an extended period. In 
central New York, overwintered females begin egg-laying in early 
May and continue until mid-July. 
The low fecundity of E. canadensis may be related to its normally 
low and relatively stable population size. Over a three-year period, 
the population densities of five other chrysomelid species that feed 
on goldenrod fluctuated by at least an order of magnitude (Messina 
and Root 1980). During this same period the population of E. cana- 
densis varied less than twofold. Furthermore, during the course of 
our long-term investigations on the goldenrod fauna at several local- 
ities in central New York, we have yet to observe a host plant that 
was significantly depleted by E. canadensis. Karren (1964) has also 
noted the stable densities of Exenia populations. Le Sage (1982), 
however, reported that during 1980-81, populations of E. canaden- 
sis increased greatly over a large area in southern Canada. 
The evolutionary steps that produced the case-bearing habit are 
unclear. Since the larval case is added to the egg case, it can be 
argued that the defense originated with the female’s habit of cover- 
ing the eggs with fecal material (this may be mixed with secretions 
from the anal gland; Hinton 1981). This initial step is exhibited by 
other chrysomelids, e.g. the eumolpine, Chrysochus auratus (Fabri- 
cius). The extant species of camptosomate beetles differ in their 
manner of oviposition and egg case deposition. Some clytrine bee- 
tles lay eggs in clusters (a typical trait of non-camptosomate chry- 
somelids), with each egg connected to the substrate by a separate 
stalk (Hinton 1981). A cryptocephaline, Pachybrachis bivittatus 
(Say), apparently does not connect the egg to the substrate at all. 
Instead, the female covers the egg with fecal material while holding 
it with her hind legs, and then simply drops the egg to the ground 
(Lawson 1976). Further comparative data on the details of egg-case 
provisioning are needed to trace further the evolution of the case- 
bearing habit and the often enigmatic phylogeny of the camptoso- 
mate line (Mann and Crowson 1981). 
