Empty Cavities

Smaller insects tend to have smaller volumes of hemolymph in the spaces of their hemocoels than do larger forms, but careful comparative studies of the relative volumes in different species are scanty. The volume of hemolymph varies with the species, age, diet, developmental stage, activity, and how volume is determined. Methods of collecting hemolymph in the past have included exsanguinations, dye dilutions and C14 measurements. Each method presents different technical difficulties and can be unreliable, but exsanguinations and dye dilution in trained hands can yield similar values (Jones, 1997: p. 74).

Ancillary evidence shows that some nymphs, termites and many caterpillars contain volumes of hemolymph that are large enough to create positive transmural pressures in the hemocoel; in some even enough hemolymph under sufficient pressure to swell their bodies. Many larvae maintain a constant volume of hemolymph relative to their weight throughout larval life, and these larvae continually produce hemolymph as they grow. Neu-rosecretion of hormones by the brain stimulates the corpora cardiaca to produce a diuretic hormone, but these endocrine interrelationships are complex. Removing the neurosecretory cells by excising the corpora cardiaca and allata may not affect water balance in some species. Fluid volume in an insect may peak prior to molting, but then this volume drops quickly afterwards. Males may possess less hemolymph than females. But in general, it appears that the volume of hemolymph may be smaller in proportion to the size of the cavity in smaller insects (personal observation).

In many insects, especially the smaller ones, cavities appear 'empty' and only 'wet' inside when opened. One exception to our size rule are the seventeen-year cicadas captured in 2004 in Maryland. These cicadas, had large moist empty hemocoels and very little fluid hemolymph. As in these cicadas, microscopic preparations of the smallest forms, when carefully fixed and sectioned for light and electron microscopy, present little evidence for accumulated fluid.

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