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Henry Walter Bates FRS FLS FGS (Leicester, 8 February 1825 – London, 16 February 1892) was an English naturalist and explorer who gave the first scientific account of mimicry in animals. He was most famous for his expedition to the rainforests of the Amazon with Alfred Russel Wallace in 1848. Wallace returned in 1852, but lost his collection in a shipwreck. When Bates arrived home in 1859 after a full eleven years, he had sent back over 14,712 species (mostly of insects) of which 8,000 were new to science.

Bates was born in Leicester to a literate middle-class family. However, like Wallace, T.H. Huxley, and Herbert Spencer, he had no formal education in science, and taught himself mainly by reading (i.e., he was largely an auto-didact). He left school at 12, and at 13 he became apprenticed to a hosier. He joined the Mechanics' Institute (which had a library), studied in his spare time, and collected insects in Charnwood Forest. In 1843 he had a short paper on beetles published in the journal Zoologist.[2]

Bates became friends with Wallace when the latter took a teaching post in the Leicester Collegiate School. Wallace was also a keen entomologist, and he had read the same kind of books as Bates had, and as Darwin, Huxley and no doubt many others had. These included Malthus on population, James Hutton and Lyell on geology, Darwin's Voyage of the Beagle, and above all, the anonymous Vestiges of the Natural History of Creation, which put evolution into everyday discussion amongst literate folk. They also read William H. Edwards on his Amazon expedition,[3] and this started them thinking that a visit to the region would be exciting, and might launch their careers.[4]

In 1847 Wallace and Bates discussed the idea of an expedition to the Amazon Rainforest, the plan being to cover expenses by sending specimens back to London. There an agent would sell them for a commission. The main purpose was for the travellers to "gather facts towards solving the problem of the origin of species", as Wallace put it in a letter to Bates. The two friends, who were both by now experienced amateur entomologists, met in London to prepare themselves. This they did by viewing South American plants and animals in the main collections.[5] Also they collected 'wants lists' of the desires of museums and collectors. Letters from the pair survive in the Kew Garden library, asking what plants the Director (then William Jackson Hooker) would like them to find.

Bates and Wallace sailed from Liverpool in April 1848, arriving in Pará (now Belém) at the end of May. For the first year they settled in a villa near the city, collecting birds and insects. After that they agreed to collect independently, Bates travelling to Cametá on the Tocantins River. He then moved up the Amazon, to Óbidos, Manaus and finally to the Upper Amazon (Solimões). Tefé was his base-camp for four and a half years. His health eventually deteriorated and he returned to England, sending his collection by three different ships to avoid the same fate as Wallace. He spent the next three years writing his account of the trip, The Naturalist on the River Amazons, widely regarded as one of the finest reports of natural history travels.

Henry Bates was one of a group of outstanding naturalist-explorers who were supporters of the theory of evolution by natural selection (Charles Darwin and Alfred Russel Wallace 1858). Other members of this group included J.D. Hooker, Fritz Müller, Richard Spruce and Thomas Henry Huxley.

Bates' work on Amazonian butterflies led him to develop the first scientific account of mimicry, especially the kind of mimicry which bears his name: Batesian mimicry. This is the mimicry by a palatable species of an unpalatable or noxious species. A common example seen in temperate gardens is the hover-fly, many of which – though bearing no sting – mimic the warning colouration of hymenoptera (wasps and bees). Such mimicry does not need to be perfect to improve the survival of the palatable species.

Bates noted of the Heliconids (long-wings) that they were forest-dwellers who were:

And yet, said Bates "I never saw the flocks of slow-flying Heliconidae in the woods persecuted by birds or dragonflies... nor when at rest did they appear to be molested by lizards, or predacious flies of the family Asilidae [robber-flies] which were very often seen pouncing on butterflies of other families... In contrast, the Pieridae (sulfur butterflies), to which Leptalis belongs [now called Dismorphia] are much persecuted."

Bates observed that a large number of the Heliconid species are accompanied in the districts they inhabit by other species (Pierids), which counterfeit them, and often cannot be distinguished from them in flight. They fly in the same parts of the forest as the model (Heliconid) and often in company with them. Local races of the model are accompanied by corresponding races or species of the mimic. So a scarce, edible species assumes the appearance of an abundant robust, noxious species. Predators learn to avoid the noxious species, and a degree of protection covers the edible species, no doubt proportional to its degree of likeness to the model. All aspects of this situation can be, and have been, the subject of research. Thus began a field of research which is still quite active today.

Bates, Wallace and Müller believed that Batesian and Müllerian mimicry provided evidence for the action of natural selection, a view which is now standard amongst biologists. Field and experimental work on these ideas continues to this day; the topic connects strongly to speciation, genetics and development.