Biology of the Arthropod Cuticle

Biochemistry and morphogenesis

Insect cuticular sclerotization: a review.

Laboratory experiments showed that cyprids of Balanus balanoides settled readily only when offered surfaces which bore settled barnacles of their own species, or the cemented bases left on surfaces from which these barnacles had been removed. Without such surfaces they proved to be capable of postponing setting for at least 2 weeks, and they chose such surfaces in preference to similar surfaces which bore related species. Cyprids of B. crenatus and Elminius modestus were also capable of making this choice.

Setting usually occurred in the immediate vicinity of barnacles or bases, but some cyprids crawled short distances away during their final reconnaissances. When they encountered barnacles attached to glass, they were stimulated to proceed with setting, but they swam away from this unfavourably smooth surface and settled on bare stones nearby.

Fragments from various parts of the body, placed upon surfaces which had never borne barnacles, made these somewhat more favourable for setting. Since attached bases alone produced a strong reaction, the cyprids probably respond to contact between their groping antennules and some substance in the cuticle. They did not settle readily when prevented from making contact with adults by a barrier of bolting silk, or with bases by a thin film of nitrocellulose; chemosensory perception of a water-soluble substance, emanating from settled barnacles, is therefore unlikely to be involved.

The power of the bases to induce setting was retained after heating to over 200° C, but not after heating to 275° C, when charring occurred. It withstood prolonged washing in water, heating with fat solvents and dilute acids, and treatment in the cold with caustic alkalis, concentrated acids, pepsin, formaldehyde, benzoquinone, sodium sulphide, phenol, urea, and diazonium solution. It was destroyed in the cold by sodium hypochlorite, which was observed to dissolve the bases, and also by hot concentrated acids and alkalis. The chemical properties of the active substance are thus entirely consistent with those of quinone-tanned proteins, which are known to form the epicuticle and attachment cement.

A field experiment showed that similar behaviour occurs under natural conditions. It leads cyprids to suitable habitats and it facilitates breeding.

Laboratory Experiments on Gregariousness During Setting in Balanus Balanoides and other Barnacles

Comparative biochemistry of the phenolase complex.

The structural and chemical changes taking place in the cuticle of the mature larva of Sarcophaga as it is converted into the puparium are fully described. The formation of the hard and dark exocuticle of the puparium from the outer endocuticle of the larva is due, as Pryor (1940 b ) has shown, to the action of a phenol derived from the blood, a polyphenol oxidase located in the inner epicuticle and present considerably before pupation being responsible for the rapid oxidation of the phenol. The blood phenol, although spontaneously oxidizable, passes unchanged through the inner endocuticle which remains soft and white, and this may be correlated with the ability of the inner endocuticle to reduce methylene blue. The phenol hardening the puparium is produced in the blood by the enzymatic oxidation of tyrosine. Tyrosine in the blood increases steadily in amount before pupation, and during this period the enzyme tyrosinase also appears and increases. The source of tyrosinase appears to lie in the oenocytoids of the blood, which increase in number as the larva matures and die away shortly before pupation. But although tyrosine and tyrosinase are present together in the blood of the mature larva, inhibiting conditions prevent the activity of the enzyme until very shortly before pupation. Tyrosinase activity, however, may be experimentally stimulated before pupation by the action of methyl alcohol and more slowly by narcotics, and it is speculated that the inhibition of tyrosinase in the mature larva may perhaps be effected by the activity of a further enzyme, a dehydrogenase, which operates by increasing the reducing power of the blood. Colorimetric and potentiometric studies of the reducing power of the blood have been carried out, and it has been found that the oxidation-reduction potential of the blood decreases steadily as the larva matures, and then rises sharply just before pupation. The coincidence of this rise with the period of liberation of the pupation hormone suggests that one function of the hormone is to destroy the reducing power of the blood, so liberating tyrosinase activity and leading to the production of a polyphenol which passes into and hardens the cuticle. In conclusion, it is suggested that the homology between the insect and crustacean cuticles is closer than has been hitherto emphasized.

/pdf/a-study-of-insect-cuticle-the-formation-of-the-puparium-of-1rnvhuxtza.pdf

A study of insect cuticle; the formation of the puparium of Sarcophaga falculata Pand. (Diptera).

The rate of growth of the larval cuticle of Sarcophaga and the chemical composition of its layers are fully described. The cuticle consists essentially of two fundamentally different layers. The outer layer--the epicuticle--is about $4\mu $ thick, and contains no chitin. It is basically composed of protein, having its isoelectric point at pH 5.1. At its surface a resistant lipo-protein complex forms a very thin membrane which may be separated intact from the remainder of the epicuticle which contains little or no lipoid. The epicuticle is therefore a double structure. The inner layer of the cuticle--the endocuticle--is much thicker than the epicuticle, and is a laminated chitin-protein complex of isoelectric point pH 3.5. Like the epicuticle it is a double structure. The outer zone of the endocuticle is secreted during early larval life, and is penetrated by pore canals which do not enter the epicuticle. The inner zone of the endocuticle is secreted later in larval life, and contains no pore canals. An exocuticle is absent from the larval cuticle, but the outer endocuticle is destined to form the exocuticle of the puparium, the inner endocuticle remaining unchanged. The pore canals at first contain cytoplasmic filaments, but these later become coiled and replaced by chitin.

/pdf/a-study-of-an-insect-cuticle-the-larval-cuticle-of-1copjrqfvf.pdf

A study of an insect cuticle; the larval cuticle of Sarcophaga falculata Pand. (Diptera).

The developing cuticle of Periplaneta shows some resistance to chemical attack even before the onset of the phenolic tanning which gives it its final hardness and colour. This early resistance is due to the impregnation of the cuticle with a lipoprotein complex. Histological and histochemical examination suggest that the combination of the sterol and protein moieties of the complex is dependent on the occurrence of some degree of aromatic bonding of the protein even before final tanning takes place. During this initial or primary aromatic bonding no marked colour is developed, and the active agent is not derived from a dihydroxyphenol, as it is in final tanning, but is a quinone formed by the oxidation in situ of the tyrosine component of the protein moiety. No free dihydroxyphenol is involved, and the protein may be referred to as ‘self-tanning’. During the process of final hardening the protein of the impregnating complex undergoes further tanning. There is some evidence that the sterol also may be involved in this process, not merely through its association with the protein, but directly owing to its combination with the tanning agent.

The cuticle of the cockroach Periplaneta americana - V. The chemical resistance of the impregnating material of the cuticle, and the 'self-tanning' of its protein component

The blood and tissues of third instar Calliphora larvae contain a glucose dehydrogenase system which shows maximum activity at the crop-full stage and declines as pupation is approached. This decline is prevented by destruction of Weismann’s ring at 3 days. It is suggested that dehydrogenase activity is the cause of the unstable reducing power of the blood and consequent inactivation of tyrosinase. The liberation of the ‘pupation hormone’ coin­cides with the complete and abrupt termination of dehydrogenase activity which leads to tyrosinase activity and hardening of the larval cuticle to form the puparium. Weismann’s ring is therefore not merely active immediately before puparium formation, but exercises a regulatory influence on developmental processes during the life of the larva. Although no direct evidence is available of the existence in blowfly larvae of a juvenile hormone such as that found in Rhodnius by Wigglesworth, the results obtained seem consistent with the presence and later elimination of such a hormone.

Weismann's ring and the control of tyrosinase activity in the larva of Calliphora erythrocephala.

The possibility that the larval cuticle of Calliphora vomitoria is capable of hydroxylating aromatic amino acids has been examined by incubation with phenylalanine and related amino acids. Hydroquinone and other dihydroxyphenols, and in addition trihydroxyphenols, have been recovered after incubation. The cuticle is also capable of forming hydroxylated products from tryptophan, making it clear that the process is non-specific.

The amino acid metabolism of a developing insect cuticle: the larval cuticle and puparium of Calliphora vomitoria. II. The non-specific hydroxylation of aromatic amino acids and the production of polyphenols by the cuticle.

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