Among the many Batesian mimetic hoverflies (Diptera: Syrphidae) some have a very precise resemblance to the presumed model (‘good’ or ‘specific’ mimics) while others have a much less precise resemblance (‘poor’ or ‘general’ mimics). Intuitively one might expect that the specific mimics would be commoner and more successful than the general mimics. However, many specific mimics (e.g. Serkomyia silentis, Volucella bombylans) are quite rare, while general mimics are common (e.g. Syrphus ribesii, Episyrphus balteatus and Eristalis intricarius). Similarly, some ant‐mimicking spiders from several different families are very good morphological and behavioural mimics of just one species of ant while others have a less detailed resemblance to ants in general. Six hypotheses are presented to explain the occurrence of so many poor mimics, and a theoretical model is outlined (the multi‐model hypothesis) which shows how a poor general mimic can have a larger population than a good, specific mimic. This hypothesis may apply to some species of hoverfly and to some ant‐mimicking spiders.
SUMMARY The protective system of eolid nudibranchs consists of crypsis, defensive behaviour patterns, autotomy of cerata, exudation of glandular secretions, ejection of nematocysts, and the structure of the epidermis. The existence of warning colouration has not been established. Different protective mechanisms are adapted to perform different functions. Epidermal vesicles probably protect the animal from explosions of nematocysts whilst feeding, and many mucous glands perform a cleansing function. Other types of gland, nematocysts, autotomy of cerata, crypsis and behaviour are all mechanisms which are directed against predators and can therefore be regarded as defensive mechanisms. Concentrated batteries of glands are described from the ceras tips of species of Catriona and of the Eubranchidae. Some of these are mucous glands, others contain protein secretions. There is evidence that these glands are important in the defence of the eolid. Nematocysts and the various types of defensive gland are probably adapted towards certain specific predators since a single mechanism is unlikely to be effective against all predators. Since the predators of different species of eolids vary, there is variation in the development of certain defensive mechanisms in these eolids. In the Aeolidiidae, Tergipes despedus, Selva rubra, Palisa papillata and perhaps in some other cleioprocts, particular types of nematocysts are stored in large numbers and these explode readily when ejected. Species of Catriona and the Eubranchidae also use nematocysts, but in most of them the concentrated batteries of glands at the ceras tip are of more importance than are the nematocysts. In Calma glaucoides nematocysts are absent, and similar batteries of glands are concentrated towards the ceras tip. In Catriona aurantia, and probably in all eolids, exudation of defensive glands and ejection of nematocysts is caused by stimulation of special neurosensory cilia which are concentrated at the tips of the cerata. Nematocysts are ejected by contraction of muscles in the cnidosac wall, and some of the defensive glands in the Eubranchidae and in Catriona species are muscle‐operated. The defensive mechanisms of eolids are concentrated in the cerata. The colour of the cerata is largely responsible for crypsis; defensive behaviour and autotomy both involve the cerata; and defensive glands and nematocysts are concentrated at the tips of the cerata. It is therefore probable that the most important function of the cerata is that of protecting the eolid against predation.
The defensive behaviour of 25 species of praying mantids from Ghana is described. All have camouflage as the primary defence mechanism, either a general resemblance to vegetation, or a specific resemblance to bark, sticks, leaves or grass. Secondary defence mechanisms include running, flying, thanatosis, secretion of fluid from the mouth, startle display and flash colouration. The early instars of several species are ant-mimics. The female Tarachodes ufielri often guards her ootheca and also the young nymphs, and she can distinguish her nymphs from intruding ants which are attacked. Some mantids have a greenhrown polymorphism and there is evidence that the frequencies of the two forms vary seasonally. Other mantids are stick, grass or bark mimics, and the possible ways in which the adaptations of these forms have evolved are outlined. The startle display is probably a bluff which protects the mantid to some extent from avian predators. I t does not appear to be directed against predaceous mammals. A curious "boxing display" is described from Oxypilus, Anasigerpes and Cutasigerpes. I t probably functions to space out individuals, not to discourage attacks from predators. The occurrence of this display is used as taxonomic evidence for the close relationship of these three genera.
Twenty‐two species of Doridacea are described from the vicinity of Dar es Salaam, Tanzania. None is new to science, but one species of the genus Gymnodoris was too immature to be fully identified. Ten of the 22 species are known from the Hawaiian Islands, indicating that many species of dorid occur throughout the Indian Ocean and the Pacific as far as Hawaii. One species (Jorunna tomentosa) appears to be cosmopolitan, and two may possibly be confined to the coast of Africa (Chromodoris annulata and C. vicind). Sixty‐nine species of dorid have already been reported from Tanzania by Eliot. Nine species in the present collection were not found by him, so 78 are now known from the area.
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