This paper is a study of the southeast Asian forms of the Anopheles hyrcanus complex with special reference to those of the Malay Peninsula, and it is to the latter forms that the detailed descriptions apply. The group is distinguished from all but a few other species by the presence in the adult female of a tuft of scales on the clypeus on each side, together with pale bands on the palps and a ventral tuft of scales on the seventh abdominal segment. For a complete diagnosis see page 6.

The usual practice has been to recognise only two oriental forms of hyrcanus: varieties sinensis and nigerrimus. It is shown that these varieties as recognised up till now are heterogeneous, and that at least in southeast Asia, ‘hyrcanus’, is a group of sibling species of which there are no less than seven or eight in the Malay Peninsula. For convenience, and where the exact species is not known, the name ‘hyrcanus’ in inverted commas is used, meaning hyrcanus sibling species group.

There is no doubt that these sympatric forms are distinct species for although individual characters, such as the width of the pale bands on the hind tarsi, are variable and the range of variation between two or more species may overlap, by using a combination of characters there is no difficulty in identifying individual adult specimens of either sex. Furthermore, offspring reared from eggs are always of the same form as the female parent. Larvae, pupae, and eggs can usually be identified, and keys are given for the identification of these stages as well as the adults.

Seven named species are described, but only one of these, A. crawfordi, is regarded as new, the others have all been described at various times, and the names later sunk as synonyms of sinensis and nigerrimus.

All seven species have been recognised also outside the Malay Peninsula, and some have a wide range from western India to the Philippines or Moluccas. Except for sinensis, the species belong to southeast Asia, where Assam, Burma, Siam, the Malay Peninsula and Sumatra support the largest number. A. sinensis also occurs in these countries, but the centre of its range is more to the northeast towards China and it does not appear to have penetrated further into the archipelago than the Malay Peninsula and Sumatra. Towards the limits of the Oriental region in northwest India and central and northern China, the southeast Asian species of hyrcanus are few or absent, and are replaced by other forms of hyrcanus of palaearctic type. In China, the name sinensis probably covers two overlapping forms, a southerly oriental one presumed to be Wiedemann's type form which lays a wide decked egg, and a northerly palaearctic form laying a narrow decked egg.

Species of which sufficient numbers have been seen from different countries show geographical variation. The effects of this variation are most marked in the relatively isolated Philippine Islands, where out of three forms occurring there, one, pseudosinensis, is regarded as a species peculiar to the Philippines, though clearly derived from the same stock as the widespread nigerrimus which does not occur there. A second, lesteri, is probably a subspecies of the form of lesteri found in Borneo and the Malay Peninsula. The group thus seems to conform to the classical pattern of speciation by geographical isolation. Much of the evolution and distribution of the species probably took place in pleistocene times.

Broadly speaking A. ‘hyrcanus’ can be described as a swamp breeder, with a preference for animal blood. But it is evident that the individual species show important differences in their biology. Some breed in sunlit waters and their larvae are often green; others seem to prefer shaded places, though not in jungle, and their larvae are usually dark coloured. A. sinensis and lesteri at least, can breed in moderately saline waters, sometimes in company with A. sundaicus and baezai.

It appears that most of the species will bite man freely out of doors shortly after dark, but are reluctant to enter rooms in search of human blood. However, the degree of reluctance to bite man probably varies to an important extent between different species. All species can usually be captured at night at cattle sheds. The adults do not often rest indoors by day, and the outdoor resting places of indiensis and lesteri are described.

In southeast Asia, though ‘ hyrcanus ’ is not one of the principal mosquito vectors of malaria or filariasis, it has been found infected on a number of occasions. In most cases the exact species cannot be determined now, but where this is possible, it has been A. nigerrimus Giles in every instance. Venhuis (1939) showed that his A. hyrcanus var. X (=nigerrimus) was a vector of malaria in Celebes and Java, and Kariadi (1941) implicated it in the transmission of malayan filariasis in southeast Borneo. In Malaya, examination of preserved specimens showed that nigerrimus was the principal or only vector during an outbreak of malaria at Kuala Lumpur which Hodgkin (1933) had shown to be due to hyrcanus, and recently nigerrimus has been found to play a part in the transmission of malayan filariasis in Kedah and Province Wellesley. Experimental infections with Wuchereria malayi, the worm which causes malayan filariasis, showed that nigerrimus was less readily infected than the principal vectors (Mansonia spp.), and some other members of the hyrcanus group, but that it bit the human carrier more readily than the latter. A. sinensis, though probably harmless in Malaya, is a vector of malaria and bancroftian filariasis in parts of China, but the exact identification of the vector is now in doubt, since there appear to be at least two forms in China included under the name sinensis.