Physiological Entomology zyxwvuts (1989) zyxwvu 14, 309-317 zyxwvu The thoracic mechanism for eclosion and digging during the extrication behaviour of Diptera JALEEL AHMAD MIYAN Department of Zoology, University of Edinburgh ABSTRACT. Dipteran flies escape from the puparium by coordinated contractions of all segments of the body. Whereas special eclosion muscles have been identified in the abdomen and head, none has previously been described in the thorax. Three pairs of large thoracic muscles are described in this paper which are involved solely with eclosion. They have ultrastruc- tures consistent with an ability to supercontract and all three degenerate within 48 h following escape from the puparium. Recordings of electrical activity show them to be rythrnically active, coincident with thoracic contractions during eclosion. Many of the non-fibrillar flight muscles are also incorporated in the eclosion motor pattern and have a precise sequence of activity. Following escape there is a rapid switch from eclosion to flight motor and this is discussed with reference to afferent mediation and changing inputs to the muscles. Key words. Diptera, eclosion, muscles, thorax degeneration. Introduction The final stage in the metamorphosis of flies involves the emergence of the adult from the puparium by eclosion (Carlson, 1977) and escape from the medium in which pupation occurred, normally by digging (Fraenkel, 1935). Eclosion and digging utilize a unique ‘extrica- tion’ motor pattern (Zdarek et al., 1984; Reid et al., 1987a, b) involving cyclical contractions and relaxations coordinated through all segments of the body. Both behaviours use the same set of motor acts of which seven components are recognized by Reid et al. (1987a)% (1) Dorsal lifting and ventral contraction of the abdomen with invagination of the abdominal sternites. (2) Longitudinal contraction of the abdomen spreading anteriorly. (3) Peristaltic relaxation Correspondence: Dr J. A. Miyan, Department of Zoology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT. and lateral contraction of the abdomen also spreading anteriorly causing a maximum exten- sion of the abdomen. Forward movement of the whole fly and maximum ptilinum extension (but not expansion) occur in this phase. (4) The peri- staltic waves of contraction in the abdomen are followed by contractions in the thorax and head. Alternating contractions occur in the right and left sides of the thorax producing ‘shoulder shrugging’ and longitudinal shortening of the thorax accompanied by expansion of the ptilinum. (5) Forward movement of the legs in pairs one after the other beginning with the ante- nor pair. (6) Relaxation of the entire fly with retraction of the ptilinum and forward rnove- ment of the head and thorax to fill the space left by the ptilinum. (7) Reappearance of the abdominal sternites signalling the end of the cycle or the beginning of a new one. The expres- sion of this motor pattern terminates immediately the fly has escaped from its confine- 309