Evolution__3rd_Edition

614 PART 5 / Macroevolution
Figure 22.1
A pair of complementary coadaptations in an ant and
caterpillar. (a) The ant (Formica fusca) is tending a caterpillar of
the lycaenid butterfly species Glaucopsyche lygdamus. The ant is
drinking honeydew, secreted by the caterpillar from a special
Interspecific coadaptation can be
experimentally tested
organ. (b) Formica fusca defending a caterpillar of G. lygdamus
against a parasitic braconid wasp. The ant has seized the wasp
in its mandibles. Bars indicate 1 mm. (Photos courtesy of
Naomi Pierce.)
22.1 Coevolution can give rise to coadaptations
between species
Figure 22.1a shows an ant (Formica fusca) feeding on the caterpillar of the lycaenid
butterfly Glaucopsyche lygdamus. The ant is not eating the caterpillar; it is drinking
“honeydew” from a special organ (Newcomer’s organ), the sole purpose of which
seems to be to provide food for ants. The reason why the caterpillars feed the ants has
been the subject of several hypotheses. Pierce & Mead (1981) carried out an experiment
which suggests that the caterpillars, at least in G. lygdamus, feed ants in return for
protection from parasites.
The caterpillars are parasitized by braconid wasps and tachinid flies. Alone, they
are almost defenseless against these lethal parasites; but the tending ants will fight off
parasites from their caterpillars (Figure 22.1b). Pierce and Mead experimentally prevented
ants from tending caterpillars. They then measured the rates of parasitism in the
experimentally unprotected and in the normally protected (control) caterpillars. Their
results show that ants reduce the rate of parasitism in G. lygdamus (Table 22.1). The
ants and caterpillars are therefore closely adapted to each other; the ants gain food, and
the caterpillars gain protection. They form a kind of interspecific coadaptation. (Here
the term “coadaptation” refers to the mutual adaptation of two species; it has also been
used to describe the mutual adaptation of genotypes (Section 8.2, p. 197) and of parts
(Section 10.3, p. 260) within an organism.) Relationships like that between ants and
lycaenids are called mutualism, many examples of which are known, and provide some
of the most charming details in natural history.
How could the coadaptation between ant and lycaenid have evolved? The morphological
structure and the behavior patterns of both ant and caterpillar appear to have
..