Salt intake

The physiological regulation of salt output (see Section 4.1.3, Excretion) is much better understood than the behavioural regulation of salt intake through feeding (Trumper and Simpson 1994). However, selection of food on the basis of salt content has been demonstrated in nymphs of Locusta migratoria: when provided with appropriate artificial diets, they will regulate salt intake, provided it does not compete with stonger regulatory mechanisms for protein and carbohydrate (Trumper and Simpson 1993; Simpson 1994). Salts have been considered either non-stimulatory or feeding deterrents in locusts, depending on concentration, but there is good experimental evidence for phagostimulatory regulation of intake in these insects, as in other animals (Simpson 1994).

A spectacular example of salt acquisition is seen in puddling behaviour of Lepidoptera, which also show sex differences in salt appetite. The habit of feeding at puddles, mud, or dung, thought to compensate for the low sodium content of larval food, is much commoner in males, and males of the skipper butterfly Thymelicus lineola (Hesperiidae) transfer large quantities of Na+ to females during mating (Pivnick and McNeil 1987). Male butterflies are also strongly attracted to nitrogen sources in decaying organic material, and the excretion of surplus water confirms that dissolved substances, rather than the water itself, are the resource acquired during puddling (Beck et al. 1999). The most striking example of Na + uptake by puddling is seen in male moths, Gluphisia septentrionis (Notodontidae), which ingest huge volumes and void the excess fluid in jets while drinking. Sodium is absorbed across the enlarged ileum of the male moths and transferred at mating to females, and then allocated to the eggs (Smedley and Eisner 1995, 1996).

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