The life history and morphology of the early stages of the yew gall midgeTaxomyia taxi(Inchbald) (Diptera: Cecidomyiidae)

Abstract

1. The majority of individuals of Taxomyia taxi develop in two years and these induce the formation of artichoke galls on yew Taxus baccata in their second year. A small number (usually <10%) mature in one year and inhabit galls which never develop further than swollen buds. The life cycle rarely (<2%) takes longer than two years. 2. Eggs are laid in early June, they hatch in one to three weeks and galls are formed in dormant buds which would produce the following year's leaf shoots. There are three larval instars between late June and mid-April of the following year (one-year life cycle) and mid-April of the next year (two-year life cycle); the pupal stage lasts six weeks and adults emerge in late May and early June. 3. The rate of development of the first instar differs in the two life cycles: about fourteen months in two-year and two months in one-year galls. After instar I, development is identical. 4. Larval weight increases most rapidly during instar II, just after eruption of the meristem into a fleshy pad, which probably enables feeding to become more efficient. The moult into the second instar does not occur until the pad erupts; this occurs in August of the first year in one-year galls and 12 months later in two-year galls. 5. A sex ratio of 73% of females was estimated from bred material. A mechanical trap operating throughout the flight period did not give reliable results. Each female can lay 157·4 ± 2·4 eggs. 6. The three larval instars can be distinguished by appearance and by measurement of width of the mandibles. Respiration is metapneustic in the first instar and peripneustic thereafter. There is no no sternal spatula in the larva and no pupal coon is formed. 7. The larval gut is simplified from that of non-gall-forming cecidomyiids. Feeding probably occurs by extra-corporeal digestion, most or all of the food being sap from the cells of the meristem and, later, the nutritive pad.