Observation and experiments have demonstrated that it is pheromones ( chemical scents ), rather than visual stimuli, which are the primary factor enabling a butterfly to recognise a potential mate. Although male butterflies will intercept almost any passing insect of approximately the same colour and size as their own species, it is the chemical messages exchanged between the sexes that stimulate courtship and copulation.

 

In many species visual recognition is negligible, and the butterflies rely almost entirely on chemical communication. South American Heliconius butterflies illustrate this point very well. The males are attracted to the scent of females that are invisible, having not yet emerged from their chrysalises. A female pupa, when close to emergence, often has several male adults in very close attendance. A frantic battle takes place the instant the female hatches, as all the males struggle to copulate with her, not even allowing her time to expand and dry her wings. The mated pair then have to endure the attention of the remaining males, which are often extremely persistent, trying to prise the pair apart. Eventually, with the approach of dusk, the unsuccessful males disperse, allowing the pair to remain copulated until the next morning.

 

Pheromones come into play not only for mate location, but also for courtship. Males of Satyrinae, Hesperiinae, Pyrginae, Theclinae, Satyrinae and some other groups have specialised scales on their forewings called "androconia", which have sacs at their bases containing pheromones. These are disseminated into the atmosphere via tiny hairs or plumes on the edges of the scales, and are used to attract females and entice them to copulate. In the case of the Grayling Hipparchia semele the male actually clasps the females antennae between his wings to bring them into direct contact with the androconial scales.

 

In Danainae the androconia are on the hindwings. The males have tufts of hair-pencils at the tips of their abdomens which they brush against the androconial scales to collect pheromones. These are later disseminated by expanding the tufts when in the presence of females.

 

Males of several Ithomiine species gather at communal "leks", where they release pheromones from hair-like androconial scales on the upperside hindwings. These attract more males, which release further pheromones. After a few days the lek may include a dozen or more different species. Passing females are attracted to the leks by the complex fragrances, and their presence stimulates the males to open their wings and release further pheromones that entice the females into copulation.

 

Courtship rituals

 

In some species the courtship process is very brief, in others it is protracted and involves complex rituals. Male and female Wood Whites Leptidea sinapis for example are often observed sitting facing each other, exchanging chemical messages with their antennae. The male repeatedly flicks out his long proboscis, whipping the female alternately on the underside of her left and right wings. Both sexes periodically flick open their wings. The butterflies are clearly communicating something but the ritual does not appear to instigate copulation, so the nature of the "messages" is unclear.

 

The courtship behaviour of butterflies in general are poorly studied, but it is clear that in most species the females will not permit males to copulate until they have completed an often complex ritual. This typically begins with the males releasing pheromones, which leads to the female settling on foliage. The male might then begin a "courtship dance" around the female, or use some other form of physical stimulus to which the female reacts in a particular way. Each stimulus must elicit the "correct" response, producing a chain reaction of events that culminates in copulation.

 

Wood Whites Leptidea sinapis engaged in courtship ritual

 

Small Tortoiseshells Aglais urticae have a protracted courtship which can last for several hours. The male follows the female as she flies from place to place, and when she settles and opens her wings, he walks onto her hindwings, tapping them with his antennae. This process is repeated many times over several hours, during which the male will drive off any other intruding males. Eventually the female leads him to a sheltered spot, typically underneath a small bush, where copulation takes place.

 

Brimstone Gonepteryx rhamni males are also commonly seen "wing-walking" on females, but this is invariably followed by the female inverting her wings and raising her abdomen - a signal to the male that she is rejecting his advances, and a sign that she has already mated ( females of most butterflies only mate once ).

 

Brimstone Gonepteryx rhamni, female raises abdomen to signal rejection to male.

 

When female Brimstones are receptive, copulation takes place almost instantly, without any observable pre-nuptial ritual. Most butterflies remain copulated only for an hour or so, but the Brimstone is quite remarkable in this respect - I once found a pair of Brimstones which remained copulated beneath a bramble leaf for an amazing 17 days before finally parting company !

 

Lysandra bellargus copulating pair ( female on left )

 

Males usually mate with several females in the course of their lives. Females of certain long-lived species such as the Monarch Danaus plexippus will mate with several males, but in the majority of species females normally only mate once. After mating, the genital opening on the females of Marsh Fritillaries Euphydryas aurinia and some other species becomes sealed, physically preventing them from mating with other males. Males of the Apollo butterfly Parnassius apollo seal the female genital opening with a structure called a sphragis, preventing other males from copulating with them.

 

Female Brimstones Gonepteryx rhamni go to the opposite extreme - they emerge in July, feed for a few weeks, and then go into hibernation for several months. After awakening the following April, they are mated, and then fly many miles, often across inhospitable terrain, stopping to lay their eggs on any buckthorn plants that they encounter on their travels. The butterflies thus spread throughout the countryside, interbreeding with other Brimstones that may have originated a considerable distance away. The resulting high genetic diversity is probably a major factor in the remarkable success of the species, which is able to exploit an enormous range of habitats and climatic extremes, being found at altitudes from sea level to 2800m, and having a range that encompasses north Africa, the whole of Europe, and extends across temperate Asia to Siberia and Mongolia.