Die Crista acustica der Tettigonioidea (Orthoptera) / The Crista acustica of the Tettigonioidea (Orthoptera)

The number of scolopidia of the crista acustica of the Tettigonioidea is species-specific. First electron microscopic investigations of these scolopidia show that each of them is composed of 4 cells: A bipolar sense cell, an enveloping cell, a scolopale cell and a cap cell. The dendrite of the sense cell terminates in a cilium which is anchored within the dendrite with a root, which divides into numerous rootlets. There are two basal bodies at the base of the cilium. The tip of the cilium projects into the cap of the cap cell.

On the Question of Nervous Syncytia: Lack of Axon Fusion in Two Insect Sensory Nerves

Electron micrographs of insect antennal nerves reveal that 98% of the nerve fibres are below 0.5 µ in diameter and are packed in bundles of naked axons without individual glia sheaths (average fibre calibre: 0.3 µ in Bombyx, 0.12 µ in Rhodnius). Re-examination of earlier light-microscopic fibre countings, which led to the hypothesis of axon fusion in insect sensory nerves, is now necessary. In the two nerves studied, each antennal sense cell is individually connected with the brain by its own axon. The results are compared with other cases of proposed axon fusion (e.g. giant fibre systems) and the physiological consequences are discussed.

The innervation of the aesthetasc hairs of Panulirus argus

The aesthetasc hairs that occur on the outer ramus of the antennule of Panulirus argus are heavily innervated. Approximately 350 neurones are present at the base of each hair, and dendrites pass up into the hair lumen. These dendrites end one-third of the distance to the hair tip. They are composed of paraciliary structures that extend beyond the surrounding sheath. The hair lumen is in contact with the external environment through a pore at the end of the hair. The sensory cells are surrounded by sheath cells along the axons and as far distally as the sensory cell nucleus. Distally the dendrites are also sheathed by glia and hypodermal cells. The distal region of the sense cell body is not invested, the cell membranes are closely applied one to another and interdigitation occurs.

The Origin of Sensory Neurones in an Insect, Rhodnius prolixus (Hemiptera)

Throughout post-embryonic growth in Rhodnius new sensory neurones are differentiated from among the ordinary epidermal cells. The four cells forming the sensillum, the trichogen, tormogen, sense cell, and neurilemma cell probably arise from the division of a single epidermal cell. These four cells are at first alike. At an early stage in their differentiation, one of them, the sense cell, gives off an axon process which grows inwards, joins the first nerve or axon which it meets, and accompanies this to the central nervous system. If established sensory nerves are interrupted by burning, the axons grow out from the peripheral end and eventually join up with an existing nerve. Sometimes they grow round and meet the nerve from which they were derived. They may then become trapped outside the basement membrane and grow in loops and circles until a stout nerve is produced which has no connexion with the central nervous system. At the imaginal moult most of the sensory hairs on the dorsum of the abdomen with their trichogen and tormogen cells disappear, but the sense cells, neurilemma cells, and axons may persist, the distal process of the sense cell ending blindly in the cuticle. Dermal glands also arise from four cells which at the earliest stage of their development are indistinguishable from the four cells of the sensilla. If the adult is induced to moult the dermal glands may develop non-articulated outgrowths resembling illformed hairs.