Salivary Glands

Salivary glands are present in most insects, though their form and function are extremely varied, and they may or may not be innervated (Ribeiro, 1995). Frequently they are known

FIGURE 16.1. Alimentary canal and associated structures of a locust. [After C. Hodge, 1939, The anatomy and histology of the alimentary tract of Locusta migratoria L. (Orthoptera: Acrididae), J. Morphol. 64:375-399. By permission of the Wistar Press.]

by other names according to either the site at which their duct enters the buccal cavity, for example, labial glands and mandibular glands, or their function, for example, silk glands and venom glands.

Typically, saliva is a watery, enzyme-containing fluid that serves to lubricate the food and initiate its digestion. Like that of humans, the saliva generally contains only carbohydrate-digesting enzymes (amylase and invertase), though there are exceptions to this statement. For example, the saliva of some carnivorous species contains protein- and/or fat-digesting enzymes only; that of bloodsucking species has no enzymes. In termite saliva there are cellulose-digesting enzymes: a p-1-4-glucanase that brings about the initial splitting of the polymer, and p-glucosidase that degrades the resulting cellobiose to glucose (Nakashima et al., 2002; Tokuda et al, 2002). (See also Section 4.2.4.)

In the innervated glands of cockroaches and locusts, release of saliva is induced when food stimulates mechano- and chemosensilla on the mouthparts and antennae. The information travels to the subesophageal ganglion and then along aminergic or peptidergic neurons to the glands where it induces relaxation of the muscles that normally close off the opening of the salivary gland duct (Ali, 1997). In contrast, the non-innervated glands of Calliphora erythrocephala are stimulated to release saliva by a hemolymph factor, possibly serotonin (Trimmer, 1985).

Other substances that may occur in saliva, though having no direct role in digestion, are important in food acquisition. For example, the saliva of aphids has a viscous component, released during penetration of the stylets, which hardens to form a leakproof seal around the mouthparts. Aphid saliva also contains pectinase and peroxidase. The former facilitates penetration of the stylets through the intercellular spaces of plant tissues while the latter may inactivate toxic phytochemicals (Miles, 1999). Hyaluronidase, which breaks down connective tissue, is secreted by some insects that suck animal tissue fluids. A spectrum of compounds that assist feeding is present in the saliva of bloodsucking species. These include anticoagulants, inhibitors of platelet disintegration, pyrase (an enzyme that breaks down ADP, to prevent platelet aggregation), and vasodilators such as nitric oxide (Ribeiro, 1995; Ribeiro and Francischetti, 2003). The nitric oxide is carried to the host's skin on heme-containing proteins (nitrophorins) (Valenzuela and Ribeiro, 1998). The nitrophorins also strongly bind histamine, released by the host to induce wound healing (Weichsel et al., 1998). Toxins (venoms), which paralyze or kill the prey, occur in the saliva of some assassin bugs (Reduviidae) and robber flies (Asilidae). It is also reported that substances that induce gall formation by stimulating cell division and elongation are present in the saliva of some gall-inhabiting species. Larvae of black flies and chironomid midges secrete large amounts of viscous saliva, forming nets that capture food particles.

In some species the glands have taken on functions quite unrelated to feeding, for example, production of cocoon silk by the labial glands of caterpillars and caddisfly larvae, and pheromone production by the mandibular glands of the queen honeybee.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment