ened and modified for jumping. They are more active
than aphids and jump and fly when disturbed. The
immature stages are usually flattened and oval; they
often have waxy strands protruding from their bod-
ies and are less active than adults.
Worldwide, there are about 2500 described species
of psyllids, in 230 genera and six families. In Costa
Rica, there are at least 250 species, with all six fami-
lies represented. Species diversity peaks in pasture/
cloud forest boundary sites at 1600-2400 m. From
sites at 1200-2000 m, 123 species are recorded, but
the number of species at these sites is undoubtedly
greater (Appendix 6). Many psyllids are free-living
and are found in clusters around the growing points
of the plants on which they live. Others induce gall
formation of a variety of shapes. Adults and later
immature stages feed by sucking sap from the phloem
tissues of their host plants, which are mainly trees and
shrubs. Each species of psyllid is highly specific to
its host and will usually only complete its life cycle
on that host. They can cause severe damage or even
kill forest tree seedlings and saplings. One un-
answered question is why certain families of plants
are heavily used as hosts (e.g., Leguminosae and
Myrtaceae) and others hardly at all (e.g., Bignoniaceae
and Rubiaceae; Appendix 6).43.6. Small Males and Large Females
in a Monteverde Scale Insect
John Alcock
Among mammals, males are often larger on average
than females of their species, sometimes to a small
degree (e.g., human beings) but sometimes extraordi-
narily so (e.g., Northern Elephant Seals [Mirounga
leonina], in which adult males weigh more than twice
as much as their mates). In contrast, most female in-
sects are larger than males of their species. Consider
the scale insect whose females can be found on the
trunks of sapling Ocotea tenera (Lauraceae) in Monte-
verde. Scale insects are known for their extraordinary
sexual dimorphism, and this species (an unnamed
species in the family Margarodidae) offers an extreme
example. The sexes are so different in size and appear-
ance that one would be forgiven for thinking that they
belonged to two completely unrelated insect species
(Fig. 4.6).
The females are large (for scale insects), thick, squat,
camouflaged brown, and wingless. They look more
like chunks of bark than an insect and nothing at all
like the tiny, delicate, fluorescent pink, winged males.
The life styles of the two sexes are correspondingly
divergent. Females spend their days pressed tight to
their host tree, feeding on the sap of the plant with
their piercing mouthparts. They can withdraw theirsucking beaks from the tree and move slowly, but
most of the time they are as sessile as barnacles. In
contrast, males probably do not feed during their pre-
sumably short lives as adults; they are ready to fly
whenever nearby females advertise their readiness
to mate, which happens between mid-afternoon and
early evening (Fig. 4.6). A receptive female announces
this by bending the tip of her abdomen upward, away
from the substrate. In this distinctive position, the
female's genital opening is exposed, which probably
allows her to release a sex pheromone (a volatile
chemical) that drifts downwind, alerting males who
are waiting on neighboring plants. The little pink males
respond quickly, zigzagging upwind along the pre-
sumptive odor plume in the manner of scent-tracking
insects.
Once a male has come close to one or more signal-
ing females, which are often surrounded by non-
receptive feeding females, he lands and searches for
one with her abdomen raised. The male positions
himself on the tip of the female's abdomen to link his
genitalia with hers. Mating continues for about 9 min
before the male dismounts and searches for other re-
ceptive females. The recently mated female slowly
lowers her abdomen, eventually adopting the stan-
dard resting position with her body flattened against
the bark of the sapling. In this position, she is unlikely
to mate again, although both males and females are
capable of copulating more than once in an afternoon
(Fig. 4.7).
Why do these margarodid scale insects (and insects
generally) exhibit large females and small males? We
recognize fundamental differences between the sexes
in their techniques for leaving descendants. Females,
whether insects or mammals, rely primarily on them-
selves in the sense that the number of offspring they
will have depends mostly on the number of eggs they
can produce. Eggs are relatively big and costly to
manufacture; large females can usually make more
eggs than small females, which drives evolution to-
ward large body size in this sex.
Males, whether insect or mammal, must find fecund
females to bear their offspring, since they typically
donate only their small, physiologically inexpensive
sperm to their progeny. Because small males can make
large quantities of tiny sperm, there is less advantage
to being large for males than for females. This pattern
breaks down when male reproductive success depends
on a male's ability to keep other males away from po-
tential mates. When access to mates depends on fight-
ing, large bodies become advantageous, and so large
male size spreads through a species over time. Huge
males of the Northern Elephant Seal win vicious fights
with other males and are able to acquire harems of fe-
males; losers do not mate at all.105 Insects and Spiders