SHORT COMMUNICATIONS 1289

reduce flight activity and therefore favour a return to the solitary state.

The present results cannot pretend to explain the com- plex phenomena responsible for the sudden expansion of swarms and the appearance of recessions. However, they may be an aid to better understanding of the role of den- sity in relation to migratory behaviour, the development of the latter during swarming, and the influence of the solitary phase on this which has often been suggested (Kennedy 1962; Lea 1968; Waloff 1976); in particular they may help towards an understanding of the alterna- tion between the solitary and the gregarious phases.

RAOUL MICHEL Laboratoire d'Ethologie, E.R.A. 489, Campus de Beaulieu, 35042--Rennes Cedex, France.

References Albrecht, F. O. 1967. Polymorphisme Phasaire et Biologie

des Acridiens Migrateurs. Pp. 180. Paris: Masson. Kennedy, J. S. 1962. The division of labour between the

phases of locusts. Colloq. Int. Cent. natl. Rech. sci., Paris, 114, 283-297.

Lea, A. 1968. Natural regulation and artificial control of Brown Locust numbers. J. entomol. Soc. South Aft., 31, 97-112.

Michel, R. 1969. t~tude au laboratoire des variations de la tendance au vol au cours du vieillissement chez le criquet p61erin Sehistocerca gregaria. Rev. Corrtp. anim., 3, 46-65.

Michel, R. 1970. l~tude exp&imentale des variations de la tendance au vol chez le criquet p61erin Schistocerca gregaria 61ev6 isol6ment pendant plusieurs g6nSra- tions. Insectes Sociaux, 17, 21-38.

Michel, R. & Albrecht, F. O. 1978. Comportement de vol et facteurs climatiques chez des Acridiens migra- teurs. Behaviour, 67, 208-216.

Michel, R., Colin, Y., Rodriguez, M. & Richard, J. P. 1977. Automatic measurement and recording of insect flight activity. Entomol. exp. Appl., 21, 199-206.

Waloff, Z. 1976. Some temporal characteristics of Desert Locust plagues. Anti-Locust Mere., 13, 1-36.

(Received: 10 March 1980; MS. number: sc-66)

Duetfing in Drosophila and Zaprionus Species Bennet-Clark et al. (Anita. Behav., 28, 230-255, 1980) have described male-female duetting in Zaprionus species and they discuss possible roles for the female songs. We have been examining sexual behaviour in the Drosophila virilis species group where the same phenomenon occurs. Zaprionus and Drosophila are very closely related genera and it is likely that some of the functions of female song will be common to both. The pattern of sexual behaviour in some members of the virilis species group makes in- vestigation of this problem easier than in Zaprionus.

We have recorded the songs of eight species within the virilis species group of which two, D. americana and D. borealis, almost invariably performed duets. In four further species, D. ezoana, D. montana, D. virilis and D. flavimontana, females sometimes sang during courtship, while in D. lacicola and D. littoralis female song was not observed. Females of the six duetting species in addition performed 'aggressive' or 'repelling' songs. Sexually im-

1 0 0 ms

Fig. 1. Courtship songs of male and female D. americana.

mature females produced these songs when courted, as did mature females when courted by more than one male at the same time, a situation females appear to find aversive. These songs are quite distinct from those pro- duced during normal courtship and it therefore seems un- likely that the latter have the same repelling function. Indeed observation clearly shows that the-female songs produced during courtship do not repel the males.

The species that we have examined in detail is D. americana, whose courtship songs are illustrated in Fig. 1. One notable characteristic of male song is a rather ir- regular amplitude modulated tone burst, while in con- trast, the female produces a more regular song consisting of trains of polycyclic pulses. Thus the female song, being more stereotyped, has more of the characteristic of a sexual isolation mechanism, a role more commonly attri- buted to male song. In support of this is the observation that in almost all D. americana courtships that we have recorded (46/48), the female started singing before the male and did so in response to male tactile stimulation. It is not until the later stages of the courtship that the male starts singing and a duet develops.

D. americana is unusual among Drosophila species in that males do not attempt to copulate until females signal their sexual readiness. The female acceptance posture is to spread the wings in high V formation; following which males usually, but not invariably, copulate with the fe- male. An acceptance posture is also found in other mem- bers of this species group. If the male does not copulate but continues to court, then the female resumes her song. This provides good circumstantial evidence that one func- tion of female song is to induce males to continue courting.

Another similarity with Zaprionus is that D. americana males continue singing after they have mounted the fe- males. The function of this behaviour is obscure; it could act to repel other males until sperm transfer is completed rather than provide further stimulation for the females.

These observations provide some evidence to support two possible ftmctions for female songs suggested by Bennet-Ctark et al. : that they are involved in sexual iso- lation and that they maintain the males' sexual interest tmtil such time as the females are prepared to mate. At the same time they refute the opinion that Zaprionus 'shares many of the advantages of Drosophila as well as offering a different and broader spectrum of behaviour'.

JOHN DONEGAN ARTHUR W, EWING

Department of Zoology, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JT.

(Received 7 May 1980; MS. number: so-70)