but at pupation it spins a floor structure that keeps
the pupa above the liquid remaining at the bottom of
the cell (Rozen 1984). After completing development,
the emerging adult must dig its way out to the main
vertical tunnel.
In the cool, windy dry season (late February), most
activity of females at La Ventana occurs between 0900
and 1430 hr (Otis et al. 1982). During the wet season
(late July) some females forage by 0600 and continue
until midafternoon. There are very few observations
of Crawfordapis foraging at flowers. Some foraging
must occur at a considerable distance from the nest-
ing site because bees are absent from their nests for
periods of 25-142 min. Detailed analyses of cell con-
tents indicate that 75-99% of the pollen in each cell
was from species of Melastomataceae, Solanaceae,
and Begoniaceae (Roubik and Michener 1985), fami-
lies that have nectarless flowers with pollen contained
in tubular anthers that open through a small pore.
Efficient collection of the pollen from such flowers
requires "buzz" pollination by bees. After landing on
a flower, a bee vibrates its flight muscles. The vibra-
tions travel through the flower, causing the expulsion
of the pollen from the anthers and onto the bee's body
(Buchmann 1983). Pollen from other plants in the
families Proteaceae, Loranthaceae, Passifloraceae,
Polygalaceae, and Sapotaceae can be found in nest
cells at low frequency and are probable sources of
nectar for female bees.
In one study (Otis et al. 1982), only about one-third
of marked bees showed high nest fidelity; most vis-
ited more than one nest, although many of these vis-
its lasted less than 1 min and represented either mis-
takes in orientation or exploratory visits to nests
already containing other females. Other bees visited
up to five different nests. In some cases, these were
young bees initiating nests. However, bees alternate
between "floater" and "provisioning" status as they
age (Jang et al. 1996). I also observed some nest switch-
ing; of the 136 bees marked in late July, 10 were still
alive on 3 September. Of these, three had become
resident in nests that were previously occupied by
other females, and another three were active in the
same holes after the intervening period of 36-40 days.
Three takeovers were observed in 90 nest holes.
Given the large investment of time and energy to
construct and provision cells, one would expect
newly emerged females to search for and take over
empty nests or nests with partially provisioned cells
(Eickwort 1975, Brockmann et al. 1979). Yet the low
rates of encounters between females and dual occu-
pancy of nests indicate that direct takeovers of occu-
pied nests occur only rarely. When a bee entered a
nest already occupied by a resident female, loud
buzzing was heard and the intruding bee left quickly.
There is no evidence of communal nesting or coop-
eration among females.
Male Crawfordapis can be readily observed near the
nesting aggregations, especially during sunny weather.
They fly over small areas, usually within 1 m of the
ground. Although they interact little with other males,
they actively pursue females returning to their nests
and attempt to mate with them. Once they leave the
nest, they may spend the remainder of their lives out-
side. They must visit flowers to obtain the nectar that
fuels their flight. Nothing is known about their foraging.
In excavations of Crawfordapis nests in Panama,
both larval and adult blister beetles, Tetraonyx cyani-
pennis (Meloidae), were found inhabiting the cells
(Roubik and Michener 1985). This species, as with
many other blister beetles, is believed to parasitize its
bee hosts when the beetle is in the first larval stage.
This long-legged larva climbs onto a flower and then
attaches to a bee as it forages. Once inside the bee's
nest, it attacks the eggs of the bee. Later larval stages
probably feed as cleptoparasites on pollen and nec-
tar stored in the cell. It is not known if this or other
blister beetles are associated with Crawfordapis.
Because Crawfordapis luctuosa has discrete nest-
ing aggregations, it offers opportunities for study.
Long-term studies of marked females would clarify
the ecological basis of nest switching and of multiple
nest visitation by some bees. By marking males as they
exit nests and observing them over time, male mat-
ing strategy could be studied. Migration rates of bees
among aggregations, rates of colonization of new nest
sites, and seasonal variation in bee activity are rich
topics for study.4.7.7. Stingless Bees of Cloud Forests
Jorge Lobo
Stingless bees (subfamily Meliponinae) are an abun-
dant group of social insects in neotropical forests.
Their colonies are organized in societies with castes
consisting of a single queen and hundreds or thou-
sands of workers. The majority of species build nests
inside hollow trees, but some nest underground,
within termite or ant nests, or in exposed construc-
tions. The common name of this group of bees comes
from their very reduced sting (functionally Stingless),
a derived condition with respect to other bees. These
bees visit numerous plant species and show wide flex-
ibility in their sources of nectar and pollen. More
characteristic is their habit of collecting materials
such as resins, gums, mineral salts, excrement, and
carrion, which are used to build internal structures
of the nest and to provide supplementary food.
In Costa Rica, there are about 50 species of sting-
less bees in 12 genera. Most species occur exclusively137 Insects and Spiders