The characteristics of the high and low density forms of noctuid moths, including Spodoptera exempta (Walker), exhibiting a density-dependent phase polyphenism have frequently been discussed in relation to migration. However, the only previous (unpublished) demonstration of an effect of larval phase on adult flight performance, using a tethered-flight technique, was invalidated by the recent discovery that the principal determinant of flight potential in S. exempta is genetic. When the incidence of prolonged flight was measured in moths derived from genetically-matched (full-sib) samples, there was a clear increase in long flights by females derived from the high-density gregaria phase larvae compared with those from solitaria phase larvae. The reasons for the apparent absence of a similar effect in males is not clear, but it is possible that the tethered-flight technique provides a less reliable index of flight capacity in this sex. The characteristics and significance of phase polyphenism in migratory noctuids are discussed. It is suggested that, in S. exempta and possibly some other comparable species, the high-density phase is adapted to accelerate re-dispersal after populations become concentrated, in order to escape the detrimental consequences of high larval densities.

SUMMARY A comparative light and electron microscope study has been made of the Nostoc colonies within the thalli of Blasia pusilla and the three British species of Anthoceros, and also of the Nostoc isolated from each bryophyte. From their morphology in culture two taxa of Nostoc have been identified; N. sphaericum from Anthoceros punctatus, A. husnotii and A. laevis, and N. calcicola from other gatherings of Anthoceros laevis and Blasia. These findings support the notion that the algal‐bryophyte associations are none specific. Within bryophyte thalli, the Nostoc trichomes are closely packed and specific features cannot be recognized. Mucilaginous sheaths are much less extensive and there is no evidence that the alga thrive in mucilage produced by the adjacent bryophyte cells. Heterocyst frequencies as high as 50% in the bryophytic Nostoc spp. strikingly recall the Azolla, Gunnera and cycad‐blue‐green algal symbioses. Akinetes, readily produced in culture were only seen in the Nostoc from dormant Anthoceros. The absence of phycobilisomes and high heterocyst frequencies in the bryophytic Nostoc spp. corroborates physiological data that the algae receive organic carbon from their hosts which enable them to fix nitrogen heterotrophically. The Nostoc colonies in both Blasia and Anthoceros are extensively penetrated by multi‐cellular bryophytic filaments. Whereas in Anthoceros these comprise highly vacuolate, thin‐walled cells, which sometimes contain intracellular Nostoc trichomes, in Blasia the filaments are thick‐walled and often develop labyrinthine wall ingrowths. This transfer cell morphology may be interpreted as an adaptation facilitating interchange of metabolites between Blasia and Nostoc. Its absence in Anthoceros perhaps indicates a far less harmonious relationship or may be related to the higher nutrient status of the habitats where the Anthoceros spp. were collected.

The giant Antarctic pycnogonid Decolopoda australis Eights takes up oxygen by diffusion across the integument, particularly of the legs. The circulatory system is feeble and haemolymph pressure changes are induced by leg movements during locomotion rather than by cardiac action. Heart rate is about30–40 beats min‐1 between 1 and 5 °C; it becomes irregular above 5 °C and ceases (reversibly) at6–7 °C. The integumentary structure appears to facilitate gaseous exchange. Although the cuticle (c. 200 μm thick) is chitinous, it is perforated (over 35% of the internal surface) by circular/ ellipsoidal tissue‐filled pits which are separated from the external environment by thin layers of chitinous cuticle no more than4–6 μm thick. The pits occur in all parts of the body and appendages (except arthrodial membranes). It is suggested that the primary function of the tissue within the pits is to form a route for gaseous diffusion, which bypasses the relatively thick impermeable chitinous layers of the rest of the cuticle. Calculations suggest that the pits reduce resistance to gaseous diffusion by about 90%. To function as a respiratory surface, the general body surface has to be kept clean. The cleaning action of the ovigers appears to be effective, except in the regions between the contiguous lateral processes of the body where detrital material accumulates.