Lepidopterans have been the object of both genetic and experimental analysis since the turn of the century (see Table 5.1). In spite of this long history of research, very little is known about the establishment of the embryonic axes during early lepidopteran development. Recent progress in the field of insect embryology has been dominated by work on the dipteran Drosophila melanogaster, due to the success of combining experimental embryological manipulations with developmental genetics and molecular biology (see Sander, Gutzeit, and Jäckle, 1985, Akam, 1987, and Ingham, 1988, for reviews). Consequently, more is known about the establishment of embryonic axes in D. melanogaster than in any other organism. Many of the genes involved in the early determination events of D. melanogaster have been cloned and their putative homologues have subsequently been isolated from a wide range of other metazoans, from vertebrates to cnidarians (see Kessel and Gruss, 1990; Murtha, Leckman, and Ruddle, 1991; McGinnis and Krumlauf, 1992). Thus the early development of the Lepidoptera and other insects formerly intractable to molecular analysis can now be studied by taking advantage of this widespread sequence homology.

In this chapter I review the literature on lepidopteran development, with an emphasis on integrating the recent molecular data with the older descriptive and experimental work. I begin with a summary of the morphological features of lepidopteran embryos as compared to other insects. I then compare what is known about development in D. melanogaster to what is known in the Lepidoptera, with an eye toward understanding the conservation of the mechanisms of segmentation and embryonic axis formation.

Morphological features

A common approach to comparing the way in which different insects pattern their early embryos is encapsulated in the categories of long, intermediate, and short germ embryos.