THE CONNECTIVE TISSUES OF INSECTSl

The significance of the connective tissue layers surrounding such or­ gans as the digestive system, Malpighian tubules, nervous system, and ova­ ries of insects has been fully appreciated only since 1950. Up to this time, it was considered that the tissues were constantly bathed by the blood and that substances dissolved in the haemolymph could pass unimpeded into the cells. This concept was, however, difficult to reconcile with the variable concentrations of common ions found in the haemolymph of different in­ sects. (26). In locusts, the potassium concentration in the haemolymph is very high, whereas the sodium concentration is very low. Since all the tis­ sues, including the nervous system are bathed in this solution, Hodgkin ( 40) concluded that conduction in insect nerves might involve a mechanism different from that in other animals. Later, however, Hoyle (42) obtained evidence which suggested that the connective tissue sheath surrounding the nervous system might act as a selectively permeable barrier, isolating the nervous system from the haemolymph. This demonstration of a possible physiological function of the connective tissue sheath created interest in the composition of the connective tissues of insects, although, as will be discussed later, it is no longer t hought that the connective tissue sheath controls the ionic composition within the nervous system. The widespread occurrence of collagenous connective tissues in insects was considered improbable. Rudall (77) suggested that the presence of large amounts of chitin in insects excludes the presence of any but minute amounts of collagen, although he did have X-ray diffraction evidence for collagen in the nerve cords of mantids. Richards (73) gave evidence from birefringence studies for the presence of collagen in the sheaths around the individual axons, but not in the neural lamella of mosquitoes. Meyer (56) considered that the fibrous elements of the connective tissues are similar to the elastic fibres of vertebrates. In the early 1950's, however, a vast amount of work was started on the collagenous connective tissues of both vertebrates and invertebrates. This work employed the newly available techniques of electron microscopy, X­ ray diffraction, and chromatography and, hence, the nature of collagenous connective tissues w�s elaborately documented within a very short time. The main characteristic of a typical collagen fibril was found to be the