(1) From a study of the anatomy and life history of Stylops, it appears that despite the existence of active winged males, fertilisation cannot occur and development is always parthenogenetic.

(2) The parasite obtains its oxygen from the outside air by means of tracheal openings on the cephalo-thorax, and it does not possess any special absorptive organs for taking up a special kind of food from the host. Nutrition appears to take place by simple filtration from the host's blood through the very thin skin of the parasite.

(3) The effect of the parasite on the internal genital organs is slight, as compared with the effect of Sacculina on Inachus, and leads to a reduction in the size of the ovaries to about quarter the normal size, while the testes are usually unaffected. The ovaries of stylopised bees never produce ripe ova, but the testes generally produce normal ripe spermatozoa.

(4) The effect on the secondary sexual characters is again slight as compared with that of Sacculina on Inachus. The external gonapophyses are usually unaltered, or they may be slightly reduced in size; the antennæ are unaltered. The scopa of the parasitised female is generally reduced in size, and she never or very rarely collects any pollen. The punctuation on the abdomen of the male may be increased.

(5) The most striking effect occurs in certain species (e.g. A. labialis and chrysosceles) in which the male normally has a yellow clypeus and the female a black one. Stylopisation in those cases may lead to the female assuming a yellow clypeus as in the male, while the male may lose the yellow and acquire a partially black clypeus.

This acquisition of the yellow clypeus by the female is the only change which can undoubtedly be interpreted as a positive acquisition of a secondary sexual character proper to the opposite sex.

(6) This effect may be brought about by male or female Stylops indifferently, the sex of the parasite having nothing to do with the nature of the effect exerted.

(7) The effects of stylopisation may be ascribed to a merely quantitative abstraction of nutriment from the gonad, leading to its partial atrophy, and not to a qualitative alteration of the metabolism such as is brought about by Sacculina. This also applies to the assumption of the yellow clypeus by stylopised females, on the analogy of the assumption of male plumage by many female birds as the result of simple ovariotomy or ovarian atrophy.

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