Courtship Behaviour in the Drosophila Obscura Group. Part Ii. Comparative Studies

[This paper describes the male courtship behaviour of 11 species of the obscura group of the genus Drosophila. These species, though differing in many details, are more like each other than any is to members of other groups. The main variation is in the form of vibration, and it is suggested that this serves to maintain species isolation. Pseudoobscura, persimilis and miranda are three closely related American species, and their behaviour differs mainly in quantitative details only. It is suggested that these differences may be related to different temperature adaptations in the natural habitat. Unlike the others, miranda frequently shows the rowing movement; this, like the wing-posture which occurs in all three species, seems to occur when the male is thwarted, but at lower intensities of courtship than wing-posture. The European species obscura, silvestris, tristis, bifasciata and ambigua follow the basic courtship pattern of the three American species, but differ in the greater frequency of rowing, and in tending to have a frontal, flatter wing-posture. It is suggested that the dark wing-tips of the male tristis help to enhance its vibration. Subobscura differs from all the others in vibrating rarely; instead, the normal prelude to copulation is wing-posture, combined with a side-to-side dance. This species relies more on visual stimuli than the others, and it is suggested that the predominance of wing-posture might be linked to this. The courtship was analysed by experiments in which all or part of the male's wings were removed; the male's courtship behaviour to wax models is also described. It is concluded that in this species there is a continuous interaction of fresh external stimuli between male and female during courtship, in contrast to the situation in pseudoobscura (and probably also the other members of the obscura group), where the male is relatively independent of such female stimulation. The variations in behaviour of these species are discussed, and some suggestions on the evolution of these changes are made. The behaviour of 2 species of the affinis division of the group - af finis and helvetica _ is briefly described. These flies court in a way very like the other, obscura-division, species, but differ particularly in the greater frequency of circling the female, and the rarity or absence of wing-posture and rowing. The origins of the courtship behaviour are discussed. It is suggested that vibration and wing-posture were derived from wing-flicking, and subsequently diverged. Rowing is thought to be derived from the fending-off movement. There is no evidence to show whether any particular form of a display can be considered primitive. The II species are classified separately on the basis of 20 courtship characters and 24 morphological ones. The classifications based on these two sets of criteria are in close agreement. The behaviour of the obscura group is compared with that of other groups. (The courtship of a melanogaster-group species, seguyi, is here described for the first time). Each group has its own specific courtship characteristics, clearly separating it from other groups. Behaviour differences are shown to be correlated with the degree of morphological divergence, from the very slight differences existing between mutants, and between sibling species, to the major ones between flies of the two great sub-genera, Drosophila and Sophophora. , This paper describes the male courtship behaviour of 11 species of the obscura group of the genus Drosophila. These species, though differing in many details, are more like each other than any is to members of other groups. The main variation is in the form of vibration, and it is suggested that this serves to maintain species isolation. Pseudoobscura, persimilis and miranda are three closely related American species, and their behaviour differs mainly in quantitative details only. It is suggested that these differences may be related to different temperature adaptations in the natural habitat. Unlike the others, miranda frequently shows the rowing movement; this, like the wing-posture which occurs in all three species, seems to occur when the male is thwarted, but at lower intensities of courtship than wing-posture. The European species obscura, silvestris, tristis, bifasciata and ambigua follow the basic courtship pattern of the three American species, but differ in the greater frequency of rowing, and in tending to have a frontal, flatter wing-posture. It is suggested that the dark wing-tips of the male tristis help to enhance its vibration. Subobscura differs from all the others in vibrating rarely; instead, the normal prelude to copulation is wing-posture, combined with a side-to-side dance. This species relies more on visual stimuli than the others, and it is suggested that the predominance of wing-posture might be linked to this. The courtship was analysed by experiments in which all or part of the male's wings were removed; the male's courtship behaviour to wax models is also described. It is concluded that in this species there is a continuous interaction of fresh external stimuli between male and female during courtship, in contrast to the situation in pseudoobscura (and probably also the other members of the obscura group), where the male is relatively independent of such female stimulation. The variations in behaviour of these species are discussed, and some suggestions on the evolution of these changes are made. The behaviour of 2 species of the affinis division of the group - af finis and helvetica _ is briefly described. These flies court in a way very like the other, obscura-division, species, but differ particularly in the greater frequency of circling the female, and the rarity or absence of wing-posture and rowing. The origins of the courtship behaviour are discussed. It is suggested that vibration and wing-posture were derived from wing-flicking, and subsequently diverged. Rowing is thought to be derived from the fending-off movement. There is no evidence to show whether any particular form of a display can be considered primitive. The II species are classified separately on the basis of 20 courtship characters and 24 morphological ones. The classifications based on these two sets of criteria are in close agreement. The behaviour of the obscura group is compared with that of other groups. (The courtship of a melanogaster-group species, seguyi, is here described for the first time). Each group has its own specific courtship characteristics, clearly separating it from other groups. Behaviour differences are shown to be correlated with the degree of morphological divergence, from the very slight differences existing between mutants, and between sibling species, to the major ones between flies of the two great sub-genera, Drosophila and Sophophora. ]