Do Flies Lie with Asymmetric Legs?

Do Flies Lie with

Asymmetric Legs?

Empidid flies are well-known for the

extraordinary variation in their mating

systems. To increase their chances of

securing a mating, the males of most

species donate prey items, captured

flies of other species, as nuptial gifts

to females  However, empidid flies

show a remarkable extent of apparently

deceptive sexual signals in both sexes

 The nuptial gift often leads to

reversed sex-roles, such that females

compete for the attention of choosy

males This role reversal can be

reflected in sexually selected body

parts. The females of some species, for

instance, possess flattened scales on

their legs which, when held against the

body, increase the apparent girth of the

female’s abdomen — males show

a preference for rotund females [5]. In

Rhamphomyia longicauda, females

take this trick a step further by inflating

their abdomen using specialised

abdominal sacs . Male empidids,

however, can be especially devious: in

some species, males present the

female with a real nuptial gift, an edible

insect, while the males of other species

present their mates with dried insect

remains or inedible items wrapped in

silk. In some species, the males take

this deceit further and entice females

using an entirely empty balloon of silk

Even in species which offer

genuine prey gifts, males sometimes

cheat by using an inedible ball of willow

fluff as a substitute gift, such as in

Empis opaca  Female Rhamphomyia

sulcata can be experimentally induced

to mate with males whose nuptial gift

has been replaced by a cotton ball .

Now, a recently discovered species

of empidid fly from the slopes of Mount

Fuji, Empis jaschhoforum, provides an

extraordinary addition to the list of

apparently deceptive traits in empidid

flies . The new species shows

remarkable and previously

undescribed variation in a male sexual

ornament, with some males being

unornamented while others can

sport ornaments on either or both

forelegs . How this extraordinary

variation is maintained by evolution

in this species is currently not

understood, but one intriguing

possibility is that it could reflect

antagonistic frequency-dependent

evolution between males and females

in a sexual system driven by cheating.

When collecting exemplars of the

new species, Daugeron et al.  found

that some males of E. jaschhoforum

possess greatly enlarged tarsi

(‘foot’ segments) on the first pair of legs,

fringed with long hairs, which probably

mimic a prey gift. Other species of

empidids are also known to have

clubbed feet which resemble males

holding prey items. However, what is

remarkable about the new species is

that the possession of enlarged foreleg

tarsi was found to vary greatly between

individual males: in one of 33 males

sampled, both tarsi were enlarged;

in 14, only one, either right or left, was

enlarged, while the remaining 18 males

showed no modification at all (Figure 1).

How could such an unusually high

level of polymorphism and asymmetry

be maintained? The authors ruled out

the possibility that the asymmetrical

males were gynandromorphs (mosaic

animals containing male and female

parts of the body), as no males

possessed other female characters.

Partial feminisation due to infection

by parasitic nematodes was also

thought unlikely. Moreover, differences

in body size between males did not

appear to account for the extent of

expression of the secondary sexual

traits, as occurs in some species

— males with modified legs were no

larger than those with un-modified legs.

Daugeron et al.  thus suggest that

a type of disruptive selection could

favour both males with the enlarged

tarsi, which may be better at attracting

females from a distance, and males with

unmodified legs, who may be subject to

less drag and be able to impress

females at close range with better

aerobatic skills . Alternatively, if the

tarsi do mimic males carrying genuine

nuptial gifts, frequency-dependent

selection might act. In general,

mimicking strategies work better when

the mimics occur at a lower frequency

than the model they are mimicking, as in

classic Batesianmodel–mimic system