5.1. Bringing the sexes together
Insects often are at their most conspicuous when synchronizing the time and place for mating. The flashing lights of fireflies, the singing of crickets, and cacophony of cicadas are spectacular examples. However, there is a wealth of less ostentatious behavior, of equal significance in bringing the sexes together and signaling readiness to mate to other members of the species. All signals are species-specific, serving to attract members of the opposite sex of the same species, but abuse of these communication systems can take place, as when females of one predatory species of firefly lure males of another species to their death by emulating the flashing signal of that species.
Swarming is a characteristic and perhaps fundamental behavior of insects, as it occurs amongst some insects from ancient lineages, such as mayflies and odonates, and also in many “higher” insects, such as flies and butterflies. Swarming sites are identified by visual markers (Fig. 5.1) and are usually species-specific, although mixed-species swarms have been reported, especially in the tropics or subtropics. Swarms are predominantly of the male sex only, though female-only swarms do occur. Swarms are most evident when many individuals are involved, such as when midge swarms are so dense that they have been mistaken for smoke from burning buildings, but small swarms may be more significant in evolution. A single male insect holding station over a spot is a swarm of one — he awaits the arrival of a receptive female that has responded identically to visual cues that identify the site. The precision of swarm sites allows more effective mate-finding than searching, particularly when individuals are rare or dispersed and at low density. The formation of a swarm allows insects of differing genotypes to meet and outbreed. This is of particular importance if larval development sites are patchy and locally dispersed; inbreeding would occur if adults did not disperse.
In addition to aerial aggregations, some male insects form substrate-based aggregations where they may defend a territory against conspecific males and/or court arriving females. Species in which males hold territories that contain no resources (e.g. oviposition substrates) important to the females and exhibit male— male aggression plus courtship of females are said to have a lek mating system. Lek behavior is common in fruit flies of the families Drosophilidae and Tephritidae. Polyphagous fruit flies should be more likely to have a lek mating system than monophagous species because, in the latter, males can expect to encounter females at the particular fruit that serves as the oviposition site.
Insects that form aerial or substrate-based mating aggregations often do so on hilltops, although some swarming insects aggregate above a water surface or use landmarks such as bushes or cattle. Most species probably use visual cues to locate an aggregation site, except that uphill wind currents may guide insects to hilltops.
In other insects, the sexes may meet via attraction to a common resource and the meeting site might not be visually located. For species whose larval development medium is discrete, such as rotting fruit, animal dung, or a specific host plant or vertebrate host, where better for the sexes to meet and court? The olfactory receptors by which the female dung fly finds a fresh pile of dung (the larval development site) can be employed by both sexes to facilitate meeting.
Another odoriferous communication involves one or both sexes producing and emitting a pheromone, which is a chemical or mixture of chemicals perceptible to another member of the species (section 4.3.2). Substances emitted with the intention of altering the sexual behavior of the recipient are termed sex pheromones. Generally, these are produced by the female and announce her presence and sexual availability to conspecific males. Recipient males that detect the odor plume become aroused and orientate from downwind towards the source. More and more insects investigated are found to have species-specific sex pheromones, the diversity and specificity of which are important in maintaining the reproductive isolation of a species.
When the sexes are in proximity, mating in some species takes place with little further ado. For example, when a conspecific female arrives at a swarm of male flies, a nearby male, recognizing her by the particular sound of her wingbeat frequency, immediately copulates with her. However, more elaborate and specialized close-range behaviors, termed courtship, are commonplace.
Swarms of both the empidids and the mosquitoes form near conspicuous landmarks, including refuse heaps or oil drums that are common in parts of the tundra. Within the mating swarm (upper left), a male empidid rises towards a female hovering above, they pair, and the prey is transferred to the female; the mating pair alights (lower far right) and the female feeds as they copulate. Females app ear to obtain food only via males and, as individual prey items are small, must mate repeatedly to obtain sufficient nutrients to develop a batch of eggs. (After Downes 1970)