The pattern of diel vertical migration of Chaoborus flavicans larvae in a shallow Scottish loch varies according to the instar and, in third and fourth instars, according to the season.The planktonic phase of the migration is not exclusively concerned with predation, the larvae feeding at least equally actively in the benthos. Although there is no conclusive evidence as to the role of vertical migration in Chaoborus flavicans, it has some of the characteristics of an epideictic display.Upward and downward locomotion can be induced experimentally in a plankton wheel by manipulating light intensity alone. However, the responses are complex and there is a great deal of individual variation, suggesting that additional factors are involved.

This is a study of the reproductive biology and morphology of the pituitary gland of the osteoglossid fish Scleropages J O F~O S U S (Muller & Schlegel). The fish spawn in the Tasek Bera waterways of Pahang, Malaysia during the rainy season beginning in August. Young fish are incubated in the mouth of the male parent until free-swimming. Males mature in their 2+ year, when 30-45 cm in length. In mature specimens the testis is small, and little sperm is produced. Some females mature in their 2+ year, although at that time, most have only reached the stage of secondary oocyte production. The chorion and follicdar granulosa of secondary oocytes are deeply invaginated into the cytoplasm. All fish spawn in their 3 + year, when 45-60 cm in length. The single ripe ovary contains 20-30 ova, each 1.9 an in diameter. No fish of 41-years were caught, which may be the result of mortality or emigration, or due to extraneous causes such as selectivity in the fishing techniques used.The pituitary gland accords with the generalized teleost pattern and embryonic development. There is no saccus vasculosus. Adrenocorticotropic cells predominate in the proadenohypophysis, and somatotropic and luteotropic cells occur here. The mesoadenohypophysis contains gonadotropic and somatotropic cells; the former grow large in ripe specimens, so that the mesoadenohypophysis becomes much enlarged. The metaadenohypophysis contains relatively few cells, much neurohypophyseal ramification, and is deeply penetrated by the third ventricle of the brain.

A study of the epideictic behaviour patterns of male Xiphophorus helleri Heckel was made, and the social rank of individual fish in 12 groups of three was assessed over a ten-day period. Straight-line hierarchies were established in most groups. An anatomical study was made to locate the adrenocortical tissue, and the adrenocortical activity of each fish was assessed by measuring mean nuclear diameter of 25 adrenocortical cells. Although results were somewhat variable, the highest-ranking fish in 7 out of 12 groups showed the least adrenocortical activity. The remaining two fish showed no significant difference from each other in adrenocortical activity, though their rank was in most cases clearly distinguishable on behavioural grounds.