Larva-adult relationships in an ancestral dipteran: A re-examination of sensillar pathways across the antenna and leg anlagen of Chaoborus crystallinus (DeGeer, 1776; Chaoboridae)

Abstract

In one of his classical studies on insect metamorphosis, Weismann compared the imaginal anlagen of the ancestral phantom midge, Chaoborus, with those of advanced brachycerans. We have expanded his findings on the relationships between larval and imaginal organs using electron microscopy and cobalt backfilling of the antenna and leg anlagen and the axonal trajectories of corresponding larval sensilla. We show that both primordia are confluent with the larval antennae and "leg" sensilla (an ancestral Keilin organ), respectively. These fully developed larval organs represent the distal tips of the imaginal anlagen rather than separate cell clusters. The axons of the larval antenna and leg sensilla project across the corresponding anlagen to their target neuromeres within the central nervous system (CNS). Within the discs, nerves composed of these larval axons, developing afferent fibres and efferences ascending from the CNS are found. Both the structure of the primordia and the axonal trajectories thus relate the situation found in advanced brachycerans with that seen in more ancestral insects. In addition, the larval antennae, legs, wings and even the eyes possess very similar afferent pioneer trajectories supporting the idea that the described pattern is generally used in the ontogeny of sensory systems.

Conservation of upstream regulators of scute on the notum of cyclorraphous Diptera

“…This provides sufficient time to construct the precise expression domains of sc required for stereotyped macrochaete arrangements. In contrast, Nematoceran species of flies have rather simple imaginal discs that are fastened down to the body wall and include only the appendages (McAlpine 1981;Melzer et al 1999;Neumann and Spindler 1991;Nishiura 2002). Nematoceran species usually have only a single size class of bristle and all sensory organs are equally spaced .…”

Development of the sensory system in larvae and pupae of Chaoborus crystallinus (DeGeer, 1776; Diptera, Chaoboridae): sensory cells, nerves and ganglia of the tail region

“…Sensillum 8duf3 appears to develop in a different way to sensilla on other appendages of the body that may be already externally visible in larvae, i.e. this is an exclusively pupal sensillum and not a larval one (Melzer et al 1999). Segment A10, however, is a highly specific larval feature that can be seen as a posterior metamere specialized for movement and possessing only a few sensilla.…”

“…The same is true for the external sensilla. Most of them, special forms of sensilla trichodea with plumose, brush-and hair-like cuticular appendages, are single-cell mechanoreceptors (Melzer et al 1999) lacking pores except for the ecdysial canal, the structure that apparently allowed DAPI to enter the body in our vital stains. Their various shapes indicate that they represent a wide segment of the mechanical waves in the water column around Chaoborus larvae and pupae (see also Giguere and Dill 1979).…”

“…This allowed us to trace the ontogeny of the sensilla and sensory projections of abdominal segments 7-10 in larvae and pupae with special reference to the reconstruction of larval sensilla and addition of new ones. Sense organs of the head and legs and their metamorphosis have been studied by Melzer and Paulus (1990, 1994 and Melzer et al (1999). The aquatic stages of Chaoborus are equipped with numerous sensory bristles which measure the mechanical waves in the water column around the animal, especially prey movements (Giguere and Dill 1979).…”

The origin and evolution of stereotyped patterns of macrochaetes on the nota of cyclorraphous Diptera

“…They are found as long folded structures at the fourth larval instar. In Clunio marinus (Chironomidae) and Chaoborus cristallinus (Chaoboridae) the discs only develop at the last larval instar (Neumann and Spindler, 1991;Svacha, 1992;Melzer et al, 1999). In Nematocera, as well as in the Orthorrapha, the appendages are fastened down to the body wall, an ancestral condition.…”