The ‘origin of larvae’ has been widely discussed over the years, almost invariably with the tacit understanding that larvae are secondary specializations of early stages in a holobenthic life cycle.

Considerations of the origin and early radiation of the metazoan phyla have led to the conclusion that the ancestral animal (=metazoan) was a holopelagic organism, and that pelago-benthic life cycles evolved when adult stages of holopelagic ancestors became benthic, thereby changing their life style, including their feeding biology.

The literature on the larval development and phylogeny of animal phyla is reviewed in an attempt to infer the ancestral life cycles of the major animal groups. The quite detailed understanding of larval evolution in some echinoderms indicates that ciliary filter-feeding was ancestral within the phylum, and that planktotrophy has been lost in many clades. Similarly, recent studies of the developmental biology of ascidians have demonstrated that a larval structure, such as the tail of the tadpole larva, can easily be lost, viz. through a change in only one gene. Conversely, the evolution of complex structures, such as the ciliary bands of trochophore larvae, must involve numerous genes and numerous adaptations.

The following steps of early metazoan evolution have been inferred from the review.

The holopelagic ancestor, blastaea, probably consisted mainly of choanocytes, which were the feeding organs of the organism. Sponges may have evolved when blastaea-like organisms settled and became reorganized with the choanocytes in collar chambers.

The eumetazoan ancestor was probably the gastraea, as suggested previously by Haeckel. It was holopelagic and digestion of captured particles took place in the archenteron. Cnidarians and ctenophores are living representatives of this type of organization. The cnidarians have become pelago-benthic with the addition of a sessile, adult polyp stage; the pelagic gastraea-like planula larva is retained in almost all major groups, but only anthozoans have feeding larvae.

Within the Bilateria, two major lines of evolution can be recognized: Protostomia and Deuterostomia. In protostomes, trochophores or similar types are found in most spiralian phyla; trochophore-like ciliary bands are found in some rotifers, whereas all other aschelminths lack ciliated larvae. It seems probable that the trochophore was the larval type of the ancestral, pelago-benthic spiralian and possible that it was ancestral in all protostomes. Most of the non-chordate deuterostome phyla have ciliary filter-feeding larvae of the dipleurula type, and this strongly indicates that the ancestral deuterostome had this type of larva.

The range of environmental sex determination and sex changes throughout plant taxa from bryophytes and pteridophytes to spermatophytes is reviewed. Lability in sex expression occurs in many plant taxa but only in homosporous pteridophytes is labile sex the rule. Among angiosperms, labile sex appears to be more common among dioecious and monoecious plants than among hermaphrodites. However, hermaphrodites can control allocation to male and female functions by varying the relative emphasis on pollen and ovules. A majority of plants with labile sex expression are perennials, which indicates that flexibility in sex is more important for species with long life cycles. Environmental stress, caused by less-than-optimal light, nutrition, weather or water conditions, often favours maleness. The extreme lability in the sex expression of homosporous pteridophytes is suggested to be related primarily to the mating systems.

Although the uses and merits of terrestrial insects as indicators have been extensively discussed, there is a lack of clear definition, goal directedness and hypothesis testing in studies in the field. In an attempt to redress some of these issues and outline an approach for further studies, three categories of terrestrial insect indicators, corresponding to differences in their application, are proposed, i.e. environmental, ecological and biodiversity indicators. The procedures in terrestrial insect bioindicator studies should start with a clear definition of the study objectives and proposed use of the bioindicator, as well as with a consideration of the scale at which the study is to be carried out. Bioindication studies are conducted at a variety of spatial and temporal scales within the context of earth-system processes, but the objectives of the study will largely determine the scale at which it would be optimally conducted. There is a tendency for studies to be conducted below their space-time scaling functions, giving them apparent predictability. The selection of potential indicator taxa or groups is then based on a priori suitability criteria, the identification of predictive relationships between the indicator and environmental variables and, most importantly, the development and testing of hypotheses according to the correlative patterns found. Finally, recommendations for the use of the indicator in monitoring should be made. Although advocating rigorous, long-term protocols to identify indicators may presently be questionable in the face of the urgency with which conservation decisions have to be made, this approach is critical if bioindicators are to be used with any measurable degree of confidence.