Cockroache; Ecology, behavior & history - W.J. Bell

easy-to-digest diet, thereby relieving them of the necessity
of finding and processing their own food. Because the
mother can meet at least part of the metabolic demands
of “lactation” from her own bodily reserves, these cockroach
juveniles are unaffected by temporary shortages of
food items in the habitat during their phase of most rapid
growth (Pond, 1983). The cockroach ability to store and
mobilize nitrogenous materials via symbiotic fat body
flavobacteria may be the basis for the variety of different
food materials offered in parental provisioning (Nalepa
and Bell, 1997, Chapter 5).
Altricial Development
After a parental lifestyle evolves, subsequent developmental
adaptations often occur that reduce the cost of
care and increase the dependency of offspring (Trumbo,
1996; Burley and Johnson, 2002). This is a universal
trend, in that the developmental correlates of parental
care are similar in both vertebrates and invertebrates. The
pampered juveniles in these parental taxa are altricial,
which in young cockroaches is evident in their blindness,
delicate exoskeleton, and dependence on adults for food
(Nalepa and Bell, 1997). Neonates of Cryptocercus are a
good example of altricial development in cockroaches.
First instars lack compound eyes; eye pigment begins developing
in the second instar. The cuticle is pale and thin,
with internal organs clearly visible through the surface of
the abdomen. Gut symbionts are not established until the
third instar, making young nymphs dependent on adults
for food. First instars are small, averaging just 0.06% of
their final adult dry weight. The small size of neonates is
associated with the production of small eggs by the female.
The length of the terminal oocyte is 5% of adult
length, contrasting with 9–16% exhibited by six other
species of oviparous cockroaches (Nalepa, 1996). Young
nymphs of Perisphaerus also lack eyes; in one species at
least the first two instars are blind (Roth, 1981b). We have
little information on developmental trends in those
cockroach species where females carry nymphs. It would
be intriguing, however, to determine if, like marsupials,
internal gestation in these species is truncated, with
nymphs completing their early development in the female’s
external brood chamber.
Juvenile Mortality and Brood Reduction
Overall, insects that exhibit parental care may be expected
to show low early mortality when compared to nonparental
species (Itô, 1980). This pattern, however, does
not seem to apply to the few species of subsocial cockroaches
for which survivorship data are available. In
Macropanesthia, mortality is about 35–40% by the time
the nymphs disperse from the nest at the fifth to sixth instar
(Rugg and Rose, 1991; Matsumoto, 1992). Both Salganea
esakii and Sal. taiwanensis incubate an average of 15
eggs in the brood sac, but average only six nymphs (third
instar) in young, field-collected families (T. Matsumoto
and Y. Obata, pers. comm. to CAN). Family size of Cryptocercus
punctulatus declines by about half during the initial
stages; a mean of 73 eggs is laid, but families average
only 36 nymphs prior to their first winter (Nalepa, 1988b,
1990). These data suggest that neonates may be subject to
mortality factors such as disease or starvation despite the
attendance of adults.
An alternative explanation for high neonate mortality
in these species is that it represents an evolved strategy for
adjusting parental investment after hatch (Nalepa and
Bell, 1997). Unlike other oviparous cockroaches, in Cryptocercus
the hatching of nymphs from the egg case is not
simultaneous, but extended in time. Hatching asynchrony
results in variation in competitive ability within a
brood, a condition particularly conducive to the consumption
of young offspring by older siblings (Polis,
1984). Nymphs of C. punctulatus 12 days old have been
observed feeding on dead siblings, and attacks by nymphs
on moribund siblings have also been noted. Age differentials
within broods may allow older nymphs to monopolize
available food, leading to the selective mortality
of younger, weaker, or genetically inferior siblings. Necrophagy
or cannibalism by adults or older juveniles may
then recycle the somatic nitrogen of the lower-quality offspring
back into the family (Nalepa and Bell, 1997). The
production of expendable offspring to be eaten by siblings
can be viewed as an alternative to producing fewer
eggs, each containing more nutrients (Eickwort, 1981;
Polis, 1981; Elgar and Crespi, 1992).
The behavioral mechanisms balancing supply (provisioning
by parents) and demand (begging or solicitation
by nymphs) are unstudied in subsocial cockroaches. In
Cryptocercus, adults appear to offer hindgut fluids periodically,
with juveniles competing for access to them. It is
probable that, like piglets, nymphs that struggle the hardest
to reach parental fluids will gain the biggest share.
Competition for food may be a proximate mechanism for
adjusting brood size and eliminating runts in other subsocial
cockroaches as well. Perisphaerus sp. females possess
just four intercoxal openings, but nine nymphs were
associated with one of the museum specimens studied by
Roth (1981b). Sibling rivalry for maternally produced
food is also observable in G. portentosa and Sal. taiwanensis
(Fig. 8.3). In Cryptocercus, there is some evidence of
parent-offspring conflict in the amount of trophallactic
food that an individual nymph receives. Adults can deny
access to hindgut fluids by closing the terminal abdominal
segments, like a clamshell. In the process of doing so
the head of a feeding nymph is sometimes trapped, and
the adult attempts to either fling it off with abdominal
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