Entomology for medical officers / by A. Alcock.

  • Alfred William Alcock
Date:
1920
Catalogue details

Licence: In copyright

Credit: Entomology for medical officers / by A. Alcock. Source: Wellcome Collection.

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    are modified, wholly or partly, for sensory purposes and for dealing with food. Behind the head the segments and appendages may be all alike, or nearly so, as in a Centipede; or the segments may be unlike and more or less fused, and their appendages diversely modified or even to some extent suppressed, as in a Crab or Insect. The cuticle or exoskeleton consists of a tough and extremely resistant substance called chitin (xircov = a coat of mail), which may be further strengthened by calcareous deposits. It is periodically moulted en masse, either throughout life, or, as in insects, during the pre-adult stages of growth. It is always thin and flexible between the segments of the body and appendages—unless they are modified by concrescence—to give freedom of movement. The Arthropod cuticle is not a true epidermis like that of Vertebrate animals, since it is not composed of cells but is merely a secretion of the true skin. The same remark applies to the “ hairs,” “ bristles,” “ spines,” and “ scales ” that clothe or fortify the bodies of many Arthropoda: all these are merely hollow outgrowths of the chitinous cuticle secreted by processes of epidermic cells; but so long as this fact is not forgotten there can be no impropriety in calling them “ hairs ” and so forth. Even those who use the terms seta (Lat. seta = bristle) and chceta (yarn? = hair) for the hairs and bristles, have no other name than “ scales ” for the broad flat setae of the wings of butterflies and mosquitoes. Not only does the cuticle form an exoskeleton, but it also sends processes inwards to form the tendons of the muscles, the lining of certain parts of the alimentary canal and genital ducts, the inner layer of the air-tubes (tracheae) when these exist, and other internal supports. The digestive, respiratory, excretory, and reproductive systems differ considerably in the different classes of Arthropoda: only the vascular and nervous systems have a general unity and constancy throughout the phylum. One general fact about the digestive system must, however, be noted ; namely, that in all normal Arthropods certain paired appendages near the mouth are differentiated, wholly or partly, to form jaws or foot-jaws.
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