Entomology for medical officers / by A. Alcock.

  • Alfred William Alcock
Date:
1920
Catalogue details

Licence: In copyright

Credit: Entomology for medical officers / by A. Alcock. Source: Wellcome Collection.

    79/404 page 59
    than the tibia. The succeeding tarsal segments decrease in length successively to the 5th, which terminates in a pair of claws: these are usually equal in size, and may be simple or may have an extra tine. The main features of the internal anatomy are shown in Fig. 10. The eggs of mosquitoes are deposited, either in masses or singly, on or very close to water, but in the case of certain species that have to adapt themselves to extreme vicissitudes of climate they may be laid in quite dry places, where, how¬ ever, they will become immersed in due season ; this, for instance, may happen with some species of Stegomyia, whose eggs when deposited in the dry season may remain dry and dormant until the advent, months afterwards, of rainy weather; it is also said to be the normal course with many sub-arctic and northern-temperate species whose eggs, deposited in autumn or even earlier, must lie dormant through the winter until the setting free of the frozen waters in spring. In some species (e.g. Culex) the eggs stick together in rows, side by side, in an upright position in “ rafts ” ; in others (e.g. Stegomyia and most Anopheles) they float singly, at full length, with but slight tendency to cohere ; in others (e.g. Chaoborus) they are imbedded in strings of jelly. The individual egg is elongate, with one end rather broader and blunter than the other, and is pigmented ; outside the chitinous shell it is coated by a delicate membrane, which may either fit it quite closely, or may invest it more loosely so as to leave on either side an air-space or float, as in Anopheles (Fig. 14). In ordinary circumstances, uncomplicated by any specific necessity for withstanding periods of prolonged drought or cold, the egg hatches within a few days—sometimes within twenty-four hours. The number of eggs laid in one batch varies considerably according to species : in Culex pipiens it ranges from about 120 to about 400, but other species may lay their eggs a few at a time over a considerable period. With those species that habitually deposit large batches the number of batches that may be produced in a lifetime is variable: in Calcutta Culex
    digitised image 1
    digitised image 2
    digitised image 3