THE author of this book ranks as the foremost American worker on insect morphology. His contributions on the subject are notable for their clarity and originality of thought, and the appearance of a volume, embodying his ideas in comprehensive form, is sure of a hearty welcome. In its preparation, Mr. Snodgrass has incorporated the results of much first-hand study with those of many recent investigators in the same field. He has produced an outstanding book wherein knowledge of facts is combined with that of function and, at the same time, theoretical conceptions of the origins and relationships of organs and parts are not overlooked. Principles of Insect Morphology
 By R. E. Snodgrass. (McGraw-Hill Publications in the Zoological Sciences.) Pp. ix + 667. (New York and London: McGraw-Hill Book Co., Inc., 1935.) 36s. net.

Principles of Insect Morphology

In insects, accumulation of yolk, or vitellogenesis, is a heterosynthetic pro­ cess, in which an extraovarian tissue, the fat body, produces yolk protein precursors ( 1 , 92). Heterosynthetic vitellogenesis occurs even in the most primitive insects, the Apterygota (9, 10, 1 13). Only for two apterygote insects have ultrastructural observations indicated participation of autosynthesis in vitellogensis (9, 10) . This claim, however, requires confirmation by biochem­ ical or immunochemical means . Because of the heterosynthetic nature of vitellogenesis in insects, their oocytes are highly specialized for the specific accumulation of extraovarian yolk protein precursors . Telfer first implicated the role of endocytosis in yolk deposition in insect oocytes . He found selective concentration of some hemolymph-borne proteins in yolk bodies of Hyalophora cecropia oocytes ( 1 29, 1 3 1 ) . Coated vesicles, the cellular structures associated with selective endocytosis, were first observed in oocytes of the mosquito Aedes aegypti ( 1 1 2) . These pioneering studies were followed by numerous electron micro­ scopical and tracer investigations that supported earlier data on the in­ ternalization pathway of extraovarian proteins in follicles and on the role of coated vesicles in yolk deposition in insect oocytes (reviewed in 1 33) .

Accumulation of yolk proteins in insect oocytes

Publisher Summary Among animal hormones, juvenile hormone (JH) is distinctive because of its unique structure and the diversity of its effects on insect development and reproduction. This chapter reviews the actions of JH on the fat body, gonads, accessory glands, muscle, and nervous system of adult insects. Whereas the epidermis is a major target of premetamorphic JH action, it has been studied little in adult insects, which generally do not moult. However, since the development of yellow pigmentation that accompanies sexual maturation in adult male locusts is clearly dependent on JH-regulation processes, in which cellular and molecular mechanism are investigated. The rapid recent progress in understanding how ecdysteroids regulate the gene activities has resulted from the opportunities afforded by drosophila melanogaster: mapped and characterized mutants, polytene chromosomes with puffs marking active genes, efficient germ-line transformation. The recognition of two aspects of JH action in the tissues of adult insects is reviewed. A model for understanding some aspects of priming by JH may be found in the action of ecdysteroids, where early genes produce factors needed for the expression of late genes. In structure, thyroxine is very different from JH, but there is considerable resemblance between thyroxine and phenoxyphenoxy carbamate, fenoxycarb. Functionally, there are marked similarities. Thyroxine governs metamorphosis in amphibians, but is remarkably pleiotropic in governing many processes ranging from the maturation of the central nervous system to thermoregulation. The chapter emphasizes the importance of selecting insect systems on the basis of their optimal features for research, rather than historical precedent or economic importance. With selection of appropriate systems and application of the cell and molecular research techniques now available, the elusive problem of JH action should soon yield to enlightenment.

Cellular and Molecular Actions of Juvenile Hormone. II. Roles of Juvenile Hormone in Adult Insects

Phylogeny is evolutionary interrelationship expressed in terms of recency of common ancestry. The elaboration of the theoretical framework of phylogenetic systematics ("cladistics") by the Hennig school [(e.g. 34, 35); for an introduction to later developments see (5)] has led to an upsurge of interest in reconstruction of phylogenies and has proved particularly stimu­ lating for inquiries into concepts of interrelationships of higher categories. This review is devoted to a treatment of the phylogeny of extant orders of insects and their closest relatives. Palaeozoic hexapods are reviewed by Wootton in this volume. The "ordinal" level has been fixed largely by convention within each major group in the Animal Kingdom. The orders recognized here are presented in Figure 1. These taxa are obviously not equivalent by any criteria, but at least it can be argued that they are all monophyletic entities. A comprehensive treatise on the phylogeny of higher hexapod categories, including lengthy discussions of previous hypotheses, was published in 1969 by Hennig (35). Hennig's views were subsequently critically assessed by Kristensen (43) with reference to a substantial amount of additional evi­ dence. These two works provide extensive bibliographies and are referred to repeatedly throughout this review. Emphasis is laid on a presentation of current views, and particular consideration is given to the numerous new proposals made in a thought-provoking, recent book by Boudreaux (15).

Phylogeny of Insect Orders

Publisher Summary This chapter discusses the structure and regulation of the corpus allatum. The corpora allata (CA) are endocrine glands in the posterior regions of the head, or in rare instances in the thorax, which are closely associated with the stomatogastric nervous system. This chapter focuses on the regulation of the CA. It also describes the embryology, innervations, and the relationships of the structure, particularly the ultrastructure, to its synthetic activity. The characteristic shape of the CA is ovoid to round but they may be elongate as in large larvae and adults of Libellula depressa. The size of the glands is frequently about the diameter of the aorta or smaller; however, there is much variation among species and even within a species, size differs with age, sex, polymorphism, and the activity cycle of the glands. Although only one type of glandular cell occurs in the CA, there are a variety of types of CA with respect to the number of cells per gland and the relative size of the cells. The CA are surrounded by a continuous noncellular basal lamina, roughly 0.1-1 μ m thick. This material occasionally projects between two glandular cells into the interior of the gland, forming trabeculae that may accompany nerves and trachea.

Structure and Regulation of the Corpus Allatum

In this paper we propose a reappraisal of the relationships between the basal hexapod lineages (the former ‘apterygote’ insects) and the other major groups of mandibulate arthropods. It results from a cladistic analysis including 72 characters based on external morphology, internal anatomy and development. Detailed comments are provided on the various characters used and the scoring of their states. The 35 terminal taxa include 12 hexapods (9 of which are basal ‘apterygote’ representatives), 7 myriapods, 13 crustaceans, and 3 chelicerates taken as outgroups. The results of our analyses are discussed in detail for each of the taxonomic groupings, and compared with those recently obtained by other authors using different approaches based on morphological, palaeontological, developmental or molecular sequence data. Our results support the monophyly of the Mandibulata, Crustacea, Atelocerata (Tracheata) and Hexapoda, but the assemblage of Myriapoda appears poorly supported. A close relationship between Crustacea and Hexapoda, as hypothesized by several authors, is not found in any of our analyses. Within Hexapoda, the Protura and the Collembola appear as independent clades, whereas the two unresolved dipluran taxa are grouped with the monophyletic Ectognatha (Archaeognatha, Zygentoma and Pterygota).

Phylogenetic relationships of basal hexapods among the mandibulate arthropods: a cladistic analysis based on comparative morphological characters

Evolution of eye structure and arthropod phylogeny

The endoskeletal structures in arthropods: cytology, morphology and evolution.

Ecdysteroids in the haemolymph and the ovaries of the firebrat Thermobia domestica (Packard) (Insecta, Thysanura): Correlations with integumental and ovarian cycles

. A review of different studies on the phylogenetic relationships of the early Hexapoda lineages shows that analyses based on molecular sequence data have led to labile and sometimes incongruous results, introducing doubt as to the reliability of the cladograms as a whole. In a recent analysis using molecular data, the Collembola, usually considered as early branching hexapods, appear to occupy a position outside the assemblage of Crustacea and Insecta, leading to the rejection of the traditional view of hexapod monophyly. However, many morphological features, as well as the results of cladistic analyses based on morphological and developmental information, contradict these conclusions. More generally, it appears that in the present state of the analytical strategies, hypotheses concerning arthropod phylogenies obtained from morphological and developmental criteria and combined analyses involving molecular and morphological data provide more reliable results than those generated by molecular information alone.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.0307-6970.2004.00261.x

Jacques Bitsch

Papers

Phylogenetic relationships of basal hexapods among the mandibulate arthropods: a cladistic analysis based on comparative morphological characters

Evolution of eye structure and arthropod phylogeny

The endoskeletal structures in arthropods: cytology, morphology and evolution.

Ecdysteroids in the haemolymph and the ovaries of the firebrat Thermobia domestica (Packard) (Insecta, Thysanura): Correlations with integumental and ovarian cycles

The phylogenetic position of early hexapod lineages: morphological data contradict molecular data