528 Experimentally, it has been demonstrated that hemocyte and humoral receptors also recognize and bind to abiotic objects, specifically, nylon, latex, and chromatography beads. Abiotic targets are recognized by their physicochemical characteristics, for example, surface charge, bead structure, and functional groups (Lackie, 1981; Lavine and Strand, 2001).

For biotic systems recognition may involve two distinct processes. First, there may be specific attraction of hemocytes to the foreign object (chemotaxis), though some authors suggest that hemocytes and foreign objects will meet randomly as a result of normal hemolymph circulation. And second, there is physical contact between hemocytes and the foreign material, central to which are the PRR. As noted, the hemocytes may have PRR built in to their plasma membrane and thus are directly able to recognize the foreign matter, or they may recognize the foreign material only after PRR in the plasma have become attached to it (Dunn, 1986). Either way, the interaction between PRR and foreign material promotes the next step in the sequence, namely, phagocytosis, nodule formation, or encapsulation.

Coagulation. Studies on hemolymph coagulation in insects lag behind those on vertebrate blood clotting for which a mass of physiological and biochemical information is available. According to Gregoire (1974), this is related largely to the extremely rapid, even instantaneous nature of hemolymph coagulation, which has hampered studies on the relative roles of hemocytes and plasma and the biochemical events that occur.

It is now accepted that all forms of coagulation involve the participation of a specific form of hemocyte, the coagulocyte, though there is still controversy as to whether other hemocyte types may also play a role in clot formation. Three basic patterns have been described on the basis of microscopical observations. Among orthopteroid insects, most Hemiptera, some Coleoptera, and some Hymenoptera, coagulation is initiated by a sometimes "explosive" discharge of small pieces of cytoplasm from the coagulocytes, though the cells remain intact. As a result, plasma surrounding the cells forms granular precipitates which gradually increase in size and density. In some Scarabaeidae (Coleoptera) and larval Lepidoptera, the coagulocytes extrude long, threadlike pseudopodia-like processes that interweave with and stick to each other to form a cytoplasmic network in which other hemocytes become trapped. Concurrently, theplasma itself gels, forming transparent elasticsheets between the processes. In some homopterans, other Coleoptera, for example, Tenebrionidae, and other Hymenoptera, clotting appears to combine the features of the first two types.

In some insects [some Heteroptera, Curculionidae (Coleoptera), and Neuroptera] hemocytes morphologically identical to coagulocytes are present, but these do not release material to induce clotting. They do, however, accumulate at wounds, and Crossley (1975) suggested that they may release bacteriostatic substances.

Limited progress has been made in understanding the biochemistry of coagulation (Bohn, 1986; Theopold et al., 2002). Early studies showing that insect plasma lacked, for example, prothrombin, thrombin, and thromboplastin, suggested that in insects this process must be very different from its counterpart in mammals. However, recent work has shown that a number of similarities do exist. For example, in insects as in mammals there are both plasma- and cellular clotting proteins (coagulogens), the latter being released as the cells rupture. Interestingly, in many insects the plasma coagulogens are lipophorins, high-molecular-weight lipoproteins synthesized in the fat body that transport lipids and hormones. The hemocyte coagulogen is a glycoprotein. Other similarities include the involvement of calcium ions, the formation of membranous vesicles (microparticles), and enzyme-controlled cross-linking of the plasma and hemocyte coagulogens.

A possible scheme for hemolymph coagulation is given in Figure 17.7. Coagulation is initiated when coagulocytes rupture, a step that requires calcium ions. This releases

FIGURE 17.7. Proposed scheme for hemolymph clotting in insects.

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