M. Tokeshi et K. Reinhardt, REPRODUCTIVE-BEHAVIOR IN CHIRONOMUS-ANTHRACINUS (DIPTERA, CHIRONOMIDAE), WITH A CONSIDERATION OF THE EVOLUTION OF SWARMING, Journal of zoology, 240, 1996, pp. 103-112
Reproductive behaviour in Chironomus anthracinus Zetterstedt (Diptera:
Chironomidae), one of the ubiquitous species in cold-temperate lakes
of Northern Europe, was investigated in Lough Neagh, Northern Ireland.
Aerial swarming is an indispensable part of mating in this species. I
t has been recognized that adults, emerging from offshore, travel to i
nshore resting sites for physical/sexual maturation, before gathering
at lake margins to form swarms. After copulation, females immediately
travel back to open-water sites (up to 1000 m offshore--the longest ov
iposition flight recorded for a chironomid species) for oviposition. P
rotandry was evident and sex ratios were completely reversed in the ne
ar-shore swarming site and the offshore oviposition site. A sequence o
f mating behaviour was clarified, involving an initial grasp, landing,
an end-to-end position, and separation. There was a high incidence of
homosexual (male-male) grasps, indicating that grasping is initiated
with a fairly crude cue, and many (heterosexual) grasps failed to proc
eed to a correct mating position. This resulted in an estimation of on
ly one out of eight grasps resulting in a proper copulation. More egg
masses were found in the surface water farther offshore, together with
spent females. An egg mass contained c. 1000 eggs and hatching occurr
ed within one month of oviposition. Sexual differences in the pattern
of reproductive migration between the emergence/oviposition site offsh
ore, the inland resting site, and the near-shore swarming site, and th
e modes of mating and mating success achieved, together point to a pos
sibility that the conspicuous swarming behaviour in this species may h
ave evolved primarily through mate selection on the part of females. A
swarm occurring at the end of the males' lives serves as an efficient
mechanism for females to select the best-quality mate, while males ha
ve no other strategy than to participate vigorously in a swarm if they
are to attain a reproductive success with choosy females. Thus, two s
exes apparently adopt different strategies in their reproductive life:
makes males spend their energy in swarming, while females pass throug
h the swarm site with a quick but guaranteed reward and spend their re
maining energy in reaching offshore sites with muddy substrata which p
rovide the best chance of survival for the offspring.