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480 NA11JRE VO!..l02 7 APRil . 1983 ------NEWS AND VIEWS------carrying different numbers of advan­ is that the rates of occurrence of favourable vation that the is accepted into the tageous mutations, as estimated from the mutations are sufficiently high that only a various colonies despite its lack of mor­ periodic selection experiments. These ex­ short interval (roughly 40 generations, on phological or chemical defences. periments showed that the average selective average) separates successive spreading Adult move freely among the advantage of a single favourable mutation events. This is relevant to theories of the , getting "food directly from workers was 0.10 for the haploid and 0.09 for the evolutionary advantages of sexual repro­ through (exchange of alimen­ diploids (not significantly different), again duction and genetic recombination, since tary liquid among members, either suggesting a high level of dominance for recombination can have no influence on reciprocally or unilaterally), by the favourable mutations. the rate of if each successive on ant larvae, and by feeding on freshly These results suggest that diploid popu­ favourable mutation becomes fixed in the killed or decomposed workers and ant lations may evolve faster than haploid population before the next one booty" 10 11 ones, at least under circumstances where appears • • 0 Using gas chromatography, Vander mutation rates limit the rate of evolution, I. Muller, H.J. in TheNewSystematics(ed. Huxley, J.S.) 185 Meer and Wojcik show that the cuticles of and Paquin and Adams suggest that this (Sys1ema1ics Associalion, London, 1940). individual beetles taken from laboratory provides an evolutionary advantage for 2. Simmons, M.J. & Crow, J .F. A . Rev. Gene/. II, 49 (1977). colonies of Solenopsis contain the 3. Kelllewell, H.B.D. The Evolution of Melanism (Oxford diploidy. This is, of course, a long-term ad­ Universily Pre-ss, 1973). specific to the particular ant vantage at the level of the population and 4. Brown, A. W .A. & Pal, R. Resistance in : each of the four Solenopsis species (WHO, Geneva, 1971). does not entail an immediate selective ad­ 5. Atwood, K., Schneider, L. & Ryan, F.J. Cold SprinR has its own distinct set of cuticular vantage to diploid variants arising in a Harb. Symp. quant. Bioi. 16, 345 ( 1951 ). hydrocarbons and its own distinct gas 6. Novick, A. & Szelard, L. Proc. natn. Acad. Sci. U.S.A. haploid population. (Another difficulty is 36, 708 (1950). chromatographic signature. Separation of that the diploid cells were larger in size and 7. Paquin, C. & Adams, J. 302,495 (1983). M. excavaticollis from any ant hosts results smaller in number than the haploid ones, so 8. Haldane, J.B.S. Proc. Camb. phil. Soc. 28,838 (1927). (after about two weeks) in loss of most of 9. Kimura, M. & Ohta, T. Theoretical Aspects of Population that the difference in rate almost disap­ Genetics (Princeton Universi ty Press, 1971). the host hydrocarbons, leaving a pattern of pears when expressed on the scale of 10. Maynard Smith, J. The 'Evolution of (Cambridge cuticular hydrocarbons that is innate to the University Press, 1978). numbers of mutations per population.) ll. Fisher. R.A. The Genetical Theor)' of Nawra/ Selection beetle and quite distinct from any one of Another important feature of these results (Oxfmd Universily Pre ss , 1930) . the four ant species. Transfer of a beetle from one to another of a dif­ ferent species results in its acquiring the Mimicry cuticular hydrocarbons of the new host: typically. 15 per cent of the beetles' cuticular hydrocarbons are found to be Beetles disguised as ants host hydrocarbons within two hours of its from Robert M. May introduction into an ant colony; the figure rises steadily to around 50 per cent within THE availability of food within colonies of particular behaviour- often regurgitation three to four days. During the first few days ants, and other social has and feeding- from their duped hosts. following introduction into an ant colony, led many other species to evolve In general, the odour of a colony of M. excavaticollis individuals are attacked, ways of life that are essentially parasitic on social insects has both innate and acquired and they respond by 'playing dead' and such colonies. Wilson 1 has given a tabular components2 • Thus ants appear to relying on their armoured exterior to pro­ summary of the 17 orders, 120 families and recognize each other by one individual tect them. hundreds of genera of arthropods that live touching its antennae to another's cuticle, In view of the diversity of cuticular as symbionts in colonies of social insects, which suggests the cuticle is a source of in­ patterns that M. ex­ and he emphasizes that this list continues to nate species-specific chemicals. The cavaticollis showed itself capable of acquir­ grow at such a rate as to suggest that only a relatively large surface of the cuticle can ing, and the manner in which such patterns small fraction of all social parasites are cur­ also serve to absorb the acquired compo­ built up following introduction of the ­ rently catalogued. For ant colonies, such nent of the colony odour from the surroun-· tle into a new ant colony, VanderMeer and 'myrmecophiles' (ant lovers) have adopted ding environment. In particular. the Wojcik conjectured that the hydrocarbons a wide variety of devices in to pass un­ cuticles of dipteran, lepidopteran and are gained passively from ant-beetle con­ noticed among their hosts; the devices are, hymenopteran species have been shown to tact, rather than being actively synthesized as it were, concrete realizations of the contain hydrocarbons that influence by the beetle. The idea was confirmed by powers for invisibility or disguise conferred species-specific behaviour in various ways. placing freshly killed M. excavaticol/is into by the Niebelung's legendary Tarnhelm. It has been suggested that such cuticular colonies of S. invicta: the corpses acquired Many species of beetle, for example, ex­ hydrocarbons function as cues for caste the characteristic hydrocarbon signature of hibit morphological mimicry, with slender and species recognition in many the ant colony within two days. body forms and colouration that match colonies3 • One way of moving unremarked Vander Meer and Wojcik's study adds their hosts. The emphasis on visual among one's hosts may thus be to syn­ one more strategy - which they call similarity is most common among parasitic thesize the appropriate cuticular hydrocar­ 'passive integration' - to the array of species that spend a lot of time with their bons. An example is provided by the beetle tricks that myrmecophiles use. Such hosts in the open (as do many beetles that Trichopsenius frosti, which is a parasite in passive acquisition of the host colony's march with army ants). Parasitic species colonies of the termite Reticulitermes odour, coupled with the beetles' armoured that live among their hosts underground flavipes; the beetle synthesizes cuticular exterior, is an effective method for cop­ tend to rely more upon breaking the hydrocarbons that are identical to those of ing with a diverse assortment of host chemical or behavioural codes of com­ its hosts4. species. 0 munication within the host species than VanderMeer and Wojcik5 have recently upon morphological similarity. Wilson' published a detailed study of the part summarizes instances where larval or adult played by cuticular hydrocarbons in the I . Wilson, f. .O . The /nse£"1 Societies Ch. 20(Har'"anJ UniH·r­ sity Pn-''·' · 1971 ). stages of myrmecophilic species produce coexistence of a myrmecophilic beetle, 2 . .Jutsum, A.K .. Saumkr.,, T .S. & ('h('rr\'11, J.M. Anim. or other chemicals that elicit Myrmecaphodius excavaticol/is, with any Behav., 27, ~39 ( 1979). 3. Howard, R.W . & Ulom~uist, G.J. A. Rn. Ent. 27, of four species of ants in the 149 (1~82) . Solenopsis (S. richteri, S. invicta, S. 4. Howard. R.\V., Md>anid, C.A. & Blomquist, G.J. Robert M. May is Class of 1877 Professor of Sdence210,431 (1980). Zoology at Princeton University, New Jersey geminata, S. xyloni). The study of colony 5. Vamkr M~..· ..· r. R.l\. & Wojcik, D.P. Sdence 218. 806 08544. odours was largely motivated by the obser- (1982i.

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