are reared together under the care of their mother, who
embeds a mass of 50-100 eggs into the branch tip of
a host plant (Mimosaceae) and then tends offspring
throughout their nymphal development. Maternal in-
vestment includes warding off potential predators
(reduviid bugs, ants, wasps, and beetle larvae) and
parasitoids (e.g., dryinid wasps), and drilling holes
into the host branches to facilitate feeding of offspring
(Wood 1974, Masters et al. 1994). Nymphs crowd to-
gether on their natal branch until they undergo a final
molt to adulthood, the sons generally molting before
daughters. Males of both species are smaller than
females.
Habitat use and population density differ in the
two species. Umbonia ataliba, which is found on the
steep slopes that border the Rio Guacimal (1300-1500
m), lives in the understory on young branches ofPithe-
cellobium and Inga trees. The density of U. ataliba in
Monteverde is low throughout the year, with family
groups widely spaced (ca. 1 per 2000 m^2 ). In contrast,
U. crassicornis, found along highly disturbed banks
on the Rio Guacimal (1000 m), is abundant in the rainy
season, living on rapidly growing branches of Acacia,
Enter'olobium, and Calliandra species. In contrast to
U. ataliba, multiple families of U. crassicornis occupy
the same tree, often the same branch, and family
groups may be dense locally (as many as 25 have been
found on a single Enterolobium tree). Although in-
dividuals of both species may enjoy the benefits of
maternal care such as better protection from preda-
tors and facilitation of feeding, U. crassicornis may
have additional advantages associated with a more
communal lifestyle, such as increased mating oppor-
tunities. Group living also invites interesting mater-
nal care options: mothers may abandon their nymphs
to the care of another attending female, since they are
capable of producing more than one brood.
Behavior of these two species differs strikingly.
Siblings of U. ataliba remain on their natal branch
well after they reach sexual maturity and mate with
each other before females disperse to restart the life
cycle. Mating off the natal branch is rare. Close in-
breeding is very unusual for animals in nature, due
to negative effects on growth and survival (Rails et al.
1979, Falconer 1981). In U. ataliba, sibling matings
seem to be promoted by slow dispersal of adults, syn-
chronous sexual maturation of siblings, and a surpris-
ing propensity for siblings to copulate, even when
non-siblings of similar sizes and ages are offered as
possible mates. In contrast, U. crassicornis rarely ex-
hibits mating between siblings; instead, they disperse
soon after the final molt (Wood and Dowell 1985).
Early dispersal may be the principal mechanism for
avoiding inbreeding, as suggested by the observation
that in the rare cases where mating between siblings
occurs, siblings (for unknown reasons) remain to-
gether long past the average disappearance day (K.
Masters, N. Gerdes, and N. Heller, unpubl. data).
One consequence of mating between siblings in
U. ataliba is that family sex ratios are significantly
biased toward females, with one son produced for
every three daughters (Masters et al. 1994). Theory
predicts that when brothers compete for mates (such
as for sisters under inbreeding conditions), females
should be selected to produce only as many sons as
necessary to ensure that daughters are fertilized and
allocate their remaining resources to daughter produc-
tion (Hamilton 1967).
The disparate inbreeding/outbreeding behaviors in
the two treehoppers may result from ecological and
population genetic factors. The more pronounced dry
periods of the areas inhabited by U. crassicornis likely
contribute to seasonal host abundance and variation
in host quality and to differences in treehopper densi-
ties and aggregation tendencies. With higher popu-
lation densities, U. crassicornis families may find
outbreeding a viable, low-cost mating strategy. An-
other difference is that the habitats used by U. ataliba
tend to be dominated by one host species, which fa-
vors host specialization. Thus, inbreeding, which
more faithfully transmits genomes (including ge-
nomes adapted to specific ecological conditions) to
succeeding generations, may be selected. Inbreeding
depression may be low because affected individuals
do not reproduce and pass on the harmful genes.
Experimental crosses in which sisters of U. ataliba
mated with either their brothers or nonbrothers re-
vealed that the reproductive success of sibling-mated
females was not lower than that of their sisters who
mated with non-siblings. In contrast, inbreeding de-
pression is strong in U. crassicornis (K. Masters, un-
publ. data), which supports the concept that the mat-
ing history of the two populations differs, with U.
ataliba reflecting an inbreeding tradition. Thus, dif-
ferences in the genetic histories, population densities,
aggregation patterns, and possibly host plant relations
are likely causes for the diverse behaviors of Umbonia
species.4.3.5. Psyllids of Costa Rican Cloud Forests
David Hollis
Adult psyllids (jumping plantlice) resemble minute
cicadas and range in length from 1 to 8 mm. They have
two pairs of membranous wings that are held rooflike
over the body. They differ from other Sternorrhyncha
(aphids, whiteflies, and scale insects) in having two-
segmented tarsi that end in a pair of claws, normally
10-segmented antennae, and hind legs that are thick-104 Insects and Spiders