Butterfly density along the river went from near 0
in January and February to over 2000 in August and
September along a strip of river 100 m x 30 m. After
individually isolating pyrrolizidine alkaloids from
over 1000 butterflies, I found a surprising result:
chemical defenses declined as butterfly density in-
creased, perhaps because ithomiines arrive from the
lowlands in March filled with pyrrolizidine alkaloids
and lay eggs. The next generation emerges without
chemical protection and cannot obtain it, since pyr-
rolizidine alkaloid flowers do not become abundant
until August.
Models by Brower et al. (1970) helped assess the
effect of variable population size and chemical de-
fense in the five most common tawny ithomiine spe-
cies. If a bird eats a distasteful butterfly, it stops
eating them for a time. If a bird samples a palatable
butterfly, it continues to eat. The risk to a single but-
terfly belonging to a mimicry complex therefore de-
pends on the number of butterflies a bird can eat, the
number of butterflies the bird will skip if it eats a bad
butterfly, the number of birds, the number of butter-
flies, and the number of butterflies that taste bad. The
number of butterflies increases at the same time
chemical defense declines. If the number of birds does
not change (and it may), the risk to any one butterfly
is nearly identical for any unit of time. Different
species varied in both the proportion of chemically
defended individuals and their relative abundance
throughout the year. The model also showed that the
best-protected butterflies belonged to species both of
high abundance and with a high proportion of de-
fended individuals. The butterflies most at risk were
those of low abundance with no protected individu-
als. The most common scenario was to be common
and poorly protected.
Why do sympatric mimicry complexes exist? If a
butterfly species enters an area with a group of simi-
larly colored butterflies of high abundance and low
chemical protection, the model suggests they should
not evolve to adopt the mimicry pattern. However,
they become an excellent model for any species that
adopts their pattern. In an evolutionary game of fre-
quency dependence, natural selection favors not just
one pattern, as predicted by Miiller, but any number
of geographically overlapping patterns.
Chemical defense is variable between species,
within species, and over time. By incorporating varia-
tion, I hypothesized why all unpalatable butterflies
do not look the same. That natural selection can cre-
ate such close color convergence in 20 species in
Monteverde, given the ecological sources of variation
that affect selection, is remarkable. Another surpris-
ing finding was the low incidence of Batesian mim-
icry. Although there are several likely Batesian mim-
ics in Pefias Blancas, I never saw Batesian mimics of
tawny butterflies. A study is needed of the altitudi-
nal abundance of Batesian species and the chemical
defense of their models.4.6. Diptera: Flies4.6.1. Introduction
Paul Hanson & Brian V. Brown
Adult flies, mosquitoes, and midges (order Diptera)
are characterized by having the hind pair of wings
reduced to small club-shaped rudiments (halteres)
that function as balancing organs during flight. These
insects have complete metamorphosis, with legless
larval stages that live in a wide variety of habitats
(Oldroyd 1964). Larvae of many of the relatively
primitive families (suborder Nematocera) are aquatic
in still and running water. These include mosquitoes,
black flies, biting midges, and chironomids. Of
the approximately 140 families of Diptera recog-
nized worldwide, nearly 90 occur in Costa Rica. The
largest families in Costa Rica, each estimated to have
500 or more species, include Tipulidae, Cecidomyi-
idae, Chironomidae, Dolichopodidae, Phoridae, and
Tachinidae.
The larvae of relatively more derived families (sub-
order Brachycera) occur in concealed habitats, such
as leaf litter, fungi, live or decomposing plant or ani-
mal tissue, and dung. Some scavenging species are
common and widespread, often associated with
human habitation. One species is Ornidia obesa
(Syrphidae), a conspicuous metallic green fly often
found around latrines. A smaller fly, Megaselia
scalaris (Phoridae), has been reared from almost every
type of decaying organic matter, including remarkably
unlikely substances (e.g., paint and boot polish).
Diptera differs from the other large orders of insects
in its diversity of parasitic associations with vertebrate
animals, notably those in which the adults (usually
females) feed on blood. Mosquito populations have
increased in Monteverde with the creation of settling
ponds at the pig farm. Black flies (Simuliidae), whose
larvae occur only in running water, can be annoying
to visitors, especially in open habitats such as pas-
tures. Their bites are usually not felt, but the large,
itching welts, with a small blood spot in the middle,
are impossible to ignore. On some evenings, tiny phle-
botomine psychodids and ceratopogonid biting midges
can be a nuisance. One of the largest blood-sucking
flies in Monteverde is the long-beaked deer fly, Scione
maculipennis (Tabanidae), which continues to hum
(vibrating its wing muscles) even after landing (J.
Longino, pers. comm.).122 Insects and Spiders