The composition of insect haemolymph was last reviewed by Mullins (1985). The phylogenetic connection was pointed out early in a classic paper by Sutcliffe (1963): in the more advanced insect orders, inorganic ions tend to be replaced as the main haemolymph osmolytes by organic molecules. From being the dominant haemolymph cation, Na + is dramatically reduced in many phytophagous insects, especially in the larval stages. This may reflect the fact that sodium is a limiting element in plant tissues and varies geographically and with plant species, tissue and age (Denton 1982). A blood Na + concentration of only 0.2 mM measured in the aphid Myzus persicae was claimed by Downing (1980) to be the lowest, by an order of magnitude, for any animal. Important organic osmolytes are amino acids, trehalose, and organic acids (Mullins 1985). Haemolymph sugar concentrations can be extremely high, as in the pea aphid Acyrthosiphon pisum, which has haemolymph with a high osmolality and a trehalose concentration of 255 mM (Rhodes et al. 1997). Analysis of the haemolymph of larval Onymacris rugatipennis (Coleoptera, Tene-brionidae) showed approximately equal osmotic contributions from cations, chloride ions, amino acids, and trehalose, all of which participate in osmoregulation (Coutchie and Crowe 1979b). The hypothesis of Zachariassen (1996) concerning water balance in xeric insects should be mentioned in this context. He suggested that in herbivorous groups, such as tenebrionids, lower metabolic rates would mean lower haemolymph Na+ concentrations (due to reduced activity of the Na + /K + -ATPase), but elevated amino acid concentrations (due to reduced Na + -amino acid cotransport), and that this is not true of predators such as carabid beetles. However, it is difficult to disentangle this hypothesis from phylogenetic constraints, carabids belonging to the basal suborder Adephaga. Moreover, dietary input has a marked effect on sodium balance in the carnivorous Dytiscidae (Frisbie and Dunson 1988).
Published data on haemolymph chemistry may differ because information is derived from both whole haemolymph and cell-free plasma, and also because haemolymph is a complex and dynamic fluid, with solute concentrations affected by multiple factors such as diet, feeding, development, hydration state, and parasitism (Mullins 1985). Lettau et al. (1977) recorded cycles in haemolymph K+ activity throughout the day in free-walking cockroaches, using ion-selective microelectrodes for continuous measurements. Wide fluctuations in haemolymph osmolality are evident in field-collected insects, with variation demonstrated on a daily basis (Fig. 4.11) in caterpillars of various species (Willmer 1980) and seasonally in the alpine weta Hemideina maori (Orthoptera, Anostostomati-dae (formerly Stenopelmatidae), Ramlov 1999). The latter insect is freeze-tolerant and its high haemo-lymph levels of trehalose and proline during winter have a cryoprotectant function (Neufeld and Leader 1998) (see also Chapter 5). Protein and amino acid patterns in haemolymph are particularly dynamic (proline as a flight fuel has already been mentioned in Section 3.2.1). Holometabolous larvae accumulate abundant storage proteins called hexamerins (consisting of six identical subunits); these are synthesized in the fat body and supply amino acids for synthesis of adult tissues during metamorphosis. Arthropod haemocyanins and insect hexamerins belong to the same protein superfamily and their evolutionary relationships are discussed by Burmester et al. (1998). Other important plasma proteins are the vitellogenins synthesized by adult females for egg manufacture. Haemolymph buffering involves proteins and organic acids as well as bicarbonate (Harrison 2001).
Metamorphosis leads to loss of water and changes in the distribution of water and ions between body compartments. Dramatic changes in body mass, water content, and haemolymph volume and osmolality have been described during the pupal-adult transformation of Lepidoptera (Nicolson 1976; Jungreis et al. 1982). The haemo-lymph volume of adult Diptera, Lepidoptera, and Hymenoptera tends to be greatly reduced in preparation for flight. Flight itself may disturb water balance and haemolymph composition. Haemolymph volume increases by 30 per cent as a result of flight in P. americana (King et al. 1986) but decreases in Rhodnius prolixus, probably due to the release of diuretic factors (Gringorten and Friend 1979). However, in these occasional flyers which are better known for fast running, it may be difficult to separate responses to stress and to flight (Corbet 1991). In the honeybee, a short flight increases haemolymph volume, while exhaustive flight reduces it drastically (Skalicki et al. 1988).
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