Parasitology Today, vol.6, no. I I, I990 353 32 Bailey, L. (1969)Ann. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Appl. Biol. 63,483-+91 33 Bailey, L. (1976)Adv. VirusRes. 20,271-304 34 Dall, D.J. (1985) Ann. AppZ. Biol. 106,461468 35 Evans, H.F. and Haarap, K.A. (1981) in Virus Persistence (Mahy, B.W. J., Munson, A.C. and Darby, G.K., eds), pp 57-96, Cambridge University Press ’ 36 Gilliam, M. (1986) Apidology 17,93-100 37 Fries, I. (1988)Apidoloa 19,31%328 38 Fries, I. (1988) Apidoloa 19,343-354 39 Konig, J.P., Mallory, B. and Erickson, E.H., Jr (1986) 3. Apic. Res. 25,58-62 40 Bailey, L. (1958) Bee World 39,92-95 41 Shimanuke, H. (1978) in Homy Bee Pests, Predators and Diseases (Morse, R.A., ed.), pp 44-61, Cornell University Press 42 Robinson, G.E. and Page, R.E., Jr (1988) Nature 333,356358 43 Gary, N.E. and Page, R.E., Jr (1987) Exp. Appl. Acarol. 3, 32s305 44 Frumhoff, P.C. and Baker, J. (1988) Nature 333,358-361 45 Gilliam, M. and Taber, S. (1988)J. Znvertebr. Pathol. 52,314-325 46 Camazine, S. (1986)Ann. Entomol. Sot. Am. 79,801-803 47 Pichon, G., Prod’hon, J. and Riviirre, F. (1980) Cah. ORSTOM. St. Ent. Mkd. Parasitol. 18,27-47 48 Bliss, D.I. (1953) Biometrics 9, 176-196 49 Lanciani, C.A. and Boyett, J.M. (1980) Parasitology 81,465-475 Trypanosome Diversity in Lambwe Valley, Kenya - Sex or Selection? Reply When exploringthe variety of mechanisms that might account for the high level of diversity of trypanosome zymodemes from the Lambwe Valley, we concluded that the situation was complex and that no single mechanism was responsible. Although we favoured selection as the best explanation for some trends, we couched our conclusions with various caveats. The main disagreement that we have with Dr Glbsoll (see pages 342-343) IS on the question of timescale. Based on the probabilities involved, we believe that sexual recombination of trypanosomes is an evolutionary force and is not frequent enough to explain the patterns observed over such a short period. The one important exception seems to be the I 980 epidemic, when recombination may have been facilitated by transfer of new genetic material from a different vector population in the Busoga focus In Uganda. At present, there is a lot we do not know about the epidemiology of human sleeping sickness, particularly in terms of disease transmission cycles amongwildlife hosts. We are also quite Ignorant of the effects of decoupled cycles between llumans, cattle and wlldlife. Finally, we know very little about the effects of alternation of vectors in multlspecies tsetse communities. Conventional wisdom may not always prove fruitful in these complex situations. For example, Dr Gibson’s speculation on a causal relationship between high f y densities and the I980 epidemic has not been confirmed by recent data. Extremely high fly densities occurred in I 983-4 and I 987-8, without the occurrence of an epidemic”. In I 987-8, huge numbers of cattle died from ttypanosomiasis, but nothing unusual was na:ed in humans. At the peakoffly densities in R.umaThlcket in February I 988 (986 flies per trap per day) trypanosome infection rates in cattle were between I 0 and 30%, but very few of these infections were T. brucei(L.H. Otieno, unpublished). Why T. brucei did not expand in the tsetse population atthis time remains a mystery. We are genuinely perplexed by the unusual distribution of zymodemes among vertebrate hosts and among tsetse, but explaining this as an artefac: of analyslng two data sets is an injustice to the phenomenon. The pattern is well-supported in our cluster and discriminant analysis’, and is still present when only the recent data set is analysed3. The causes of this phenomenon need to be investigated. Similarly, it is naive to assume that zymodemes from the Lambwe Valley have been generated simply by reassorting enzyme patterns in every possible corn bination. A simple tally of the major phenotypes ofthree important enzymes used in the analysis reveals how only some of the potential corn binations are represented by the majority of isolates: of 27 such combinations from East African phenotypes, I4 are missing. As the data on J. brucei from the Lambwe Valley are historical rather than experimental, it 6 futile to try to prove alternative explanations for trypanosome diversity in the Lambwe Valley. Nevertheless, we would like to point out a few areas where research might help to differentiate between alternatives. First, defined and appropriate stocks of trypanosomes should be used to unravel the questions of compatibility and facilitation of sexual recombination, and to understand the host and vector selectivity of different genetic types. There should be no shortage of trypanosome stabilates from past collections to initiate this work. Second, we would encourage field workers to study the natural transmission dynamics of wildlife trypanosomlasis. This would be a first step towards understanding the evolutionary history ofthe disease, beforetackllngthe more complex relationships involved In cattle and human transmission cycles. As the landscape ecology of Africa changes with increasing human population pressure, there are likely to be quite urgent reasons for better understanding the processes behind genetic variation in J. brucei. Steve Mihok and Leonard H. Otieno Tsetse ResearchProgramme InternatIonal Centre of Insect Physiology and Ecology PO Box 30772 Nairobi. Kenya References I Mihok. S., Otieno, L.H. and Darji, N. (1990) Poras,tofogy 100.2 19-233 2 Otleno. L.H., Onyango, P. and Mpanga, E. ( I 990) DIscovery lnnovotron 2,97- IO2 3 Otieno, L.H., Darji, N. and Onyango. P. insect Science and its Application (In press) In Defense of Behavioural Host Location A.W. Pike expresses some doubts concerning the role played by behavioural responses to stimuli in host finding by the free-swimming stages of the copepod Salmincola edwardsii, an ectoparaslte of salmonid fishes (see pages 343-344). He argues that our study’ fails to show that increases in swimmingactivityfollowing shadowing or shock waves result in improved transmission rates in natural environments. We agree that, although difficult to perform, experiments quantifying the Impact of these responses upon transmission success would be extremely valuable. However, we also believe that the importance of these behaviour patterns cannot be denied. The sensory-motor system of the free- living stages of parasites has evolved to maximize the fitness of its owner, ie. In the case of copepodids, transmlsslon to a fish host. Increases in swimming activity as a response to stimuli have obvious costs for these short-lived, non-feeding stages: depletion of their limited energy reserves and possibly greater vlslblllty to predators, both resulting in decreased lifespan. To have been favoured by natural selectlon, the benefits of these responses must outweigh the costs. The only apparent benefit is a greater probability of making contact with a fish, and thus improved transmission success. Shadows and vibrations can be generated by a variety of organisms that are not suitable hosts for copepodids. Pike argues that in nature copepodids are under constant sensory bombardment from spurious stimuli of varying intensity emanating from numerous sources. We suggest that copepodids probably respond only to stimuli of intensities above a critical threshold2, covering the range which may be produced by fish. Other stimuli would then form background noises which fail to initiate responses. The importance of this noise would be reduced if, as we suggested earlier’, 5. edwardsii copepodids live in close association with the bottom rather than in the dense planktonic fauna as Pike implies. Furthermore, copepodlds are in a situation where they have everything to win and nothing to lose: If they do not find a host, they face unavoidable death within days. Taking some risks and expending energy