Study on Relationships between Habits and External Structures in Oligotrophidi Larvae (Diptera, Cecidomyiidae)

Relationships between larval habits and external structures in last instar larvae in the Oligotrophidi were studied. More than one hundred species distributed among approx. 25 genera were examined. For comparison, literature records of larval habits and structural features referring to other taxonomic categories of gall midges were also evaluated. – The external larval morphology in the Oligotrophidi seems in several respects to be more specialized in various species whose larvae are solitary or are pupating regularly on the host plant than in the great majority of species with gregarious larvae pupating in the soil. Thus, the lateral papillae on the thorax and the ventral papillae on the abdominal segment 8 show a reduced pattern mainly in species whose larvae are solitary. Generally, species pupating in the soil possess a well-developed spatula sternalis while various species with solitary larvae regularly pupating on the host plant show a more or less transformed spatula or no spatula at all. Mamelons on the ventral body side, probably organs facilitating locomotion, are conspicuous in most species pupating in the soil but small or lacking in many species with solitary larvae pupating on the host plant. Another difference, a body which is less elongated in several species showing solitary habits than in the remaining species studied in the present context, is possibly also connected with the mode of life. – Gall midges of other taxonomic categories of a higher rank show similar tendencies, as regards both the lateral papillae and the spatula sternalis. However, there seems to be no clear correlation between larval habits and the pattern of ventral papillae on the abdominal segment 8. Definite figures relating to mamelons or body shape in these midges are not available. – The study also yielded many examples of a large intra-specific variation in some external structures of the larvae. In many cases this variation considerably delimits the possibilities of finding consistent distinguishing characteristics at species level.