The geometry of insect pairing

Abstract

In insects bilateral symmetry is practically universal, except in some minor matters, as, for example, the slight overlap of the elytra exhibited by many beetles. In the females the symmetrical condition may be taken to be almost universal, but the males in certain families exhibit asymmetry, which in some cases is very extreme, in the terminal segments of the abdomen. It is with this condition that the present paper deals, and the simplest course to pursue is, first to state the case as shown by some species of Diptera, that being the Order most familiar to the author and one in which an asymmetrical condition is comparatively common, then to outline a possible explanation of the phenomenon, in the course of which certain terms will be defined in order to clear the ground of existing ambiguities. Certain statements made in the course of the argument will then be justified as far as possible, and, finally, a few special points will be discussed. In most insects, excluding such aberrant forms as the dragon-flies, the genital tube opens on the under side of the 9th abdominal segment, and the anal orifice is in the 10th. Let us trace a line in the vertical median plane of the insect, beginning on the dorsal side of the abdomen and proceeding round to the ventral side. On such a peregrination we shall first encounter the anal orifice and subsequently the genital one; this is true for all females and for most males, but there are some remarkable exceptions. Thus, it was shown by Snodgrass that in the Asilid genera Dascillis and Laphria a different condition exists. On referring to the figures in that paper, it will be seen that on tracing such a line round the insect in its median plane the genital orifice is met before the anal one ; the hypopygium is then said to be “inverted.” The term “hypopygium” will be used for the combination of the 9th and 10th segments, which are commonly fused into a single complex in flies, so that no movement of the one segment relative to the other is possible, and the two segments must always behave kinematically as a single body. This inversion is produced by the presence of a twist of 180° about the main axis of the body between the 6th segment and the hypopygium; the 8th segment is quite unsymmetrieal and has an axial twist of about 150°; the hypopygium has the complete twist of 180°, but nevertheless it is practically symmetrical about the median plane which still bisects it. The result of this twist is to produce a true asymmetry, although it may not be very apparent on casual examination; in fact, not until the relative positions of the orifices are looked into.