Evolutionary trends in Heteroptera

1. This work, the first volume of a series dealing with evolutionary trends in Heteroptera, is concerned with the egg system of about 400 species. The data are presented systematically in chapters 1 and 2 with a critical review of the literature after each family.2. Chapter 3 evaluates facts about each egg character. I have attempted to distinguish between anagenetic and cladogenetic processes of evolution.3. The actual chorion reveals a wide range of types of architecture and of aeropylar systems (fig. 264-267). The aerostatic inner layer of the shell of most Geocorisae and of Hydrometra is not homologous with the porous inner layer of Saldidae, many Amphibicorisae and Hydrocorisae. A thin, entirely solid chorion is considered as a plesiomorphous condition (Hebrus, Mesovelia, Idiostolus, Embiophila, Oncylocotis). Some of the specific features of shells are: air clefts; respiratory horns on the rim or on the operculum; porous structures in different stages of evolution, ultimately acting as a plastron; regulation of respiration by movable slips of the rim.4. The general trend of evolution of the micropylar system (fig. 268-270) represents multiplication and displacement of the micropyles, starting from a single micropyle in the centre of the cephalic pole; in some groups reduction and complete loss of the micropyles is associated with traumatic insemination. The structure and orientation of the micropyle(s) and their changes during evolution have been discussed. Monalocoris, Bryocoris and Oncylocotis are divergent from the general pattern.5. The primitive longitudinal dehiscence of the shell has evolved independently and along different pathways into a cap in most major groups (schemes in fig. 270-272), sometimes intermediately on one of the lateral sides. The lygaeid-coreid types of eclosion are derived from a transitional, radial structure which has consolidated in Piesmatidae, and also in Malcidae after loss of a central polygon. The terms operculum and pseudoperculum are redefined on a more functional basis. The cap of the serosal cuticle has evolved more slowly than the cap of the chorion.6. The progressive evolution in size is usually accompanied by some regularities as shown graphically in fig. 273, 274: small eggs have a spacious hexagonal pattern from the follicle; plesiomorphous species are usually small, have a long period of oviposition and few ovarioles in which a few, relatively large, eggs ripen simultaneously; apomorphous species have a larger body, smaller egg and follicle cells, more ovarioles, and synchronous egg maturation and deposition.7. A survey is given of incubation periods, diapause phenomena and reproductive cycles; the phylogenetic consequences are limited. With one exception, diapause of the egg intervenes in embryonic development between the stages of the early germ band and revolution (fig. 275), but does not affect the formation of the serosal cuticle.8. Family groups are distinguished by different types of embryogenesis. A wide range of progressive evolution of the embryonic development is apparent within most of the major phyletic lines. Altogether, the diversity of embryonic features and processes in Heteroptera is not equalled in any other Order of insects as far as is known.9. The variable characteristics utilized in reconstructing the genealogy of embryogenic patterns (fig. 276) are: degree of visible development of the 'pregerm'; location of blastopore; growth, orientation, transformation in shape and displacement (mostly clockwise rotations) of the germ band, embryo and prolarva.10. Since relations between the various ontogenies of the embryo appear to be independent of evolutionary adaptations, the phylogeny of the embryogenetic patterns gives a most reliable picture to contrast with quite different characters used for major classification.11. The archetype of embryogenesis is distinguished by invagination of the embryo (morphologically at the caudo-ventral edge of the egg) along the longitudinal median axis of the yolk column without loss of contact between head lobes and serosa and by a 1800 rotation of the embryo before revolution. Hebrus most closely conforms to this type, followed by most Amphibicorisae and by cimicoid groups which tend to invaginate at the left side of the egg. Temporary complete invagination occurs in Saldidae, Gerris and Hesperoctenes, and in diapausing mirid eggs.12. The variability in the type of egg rotation and embryo rotation in Gerridae, Hydrometridae, Cydnidae and Acanthosomatidae suggests that the egg system is not yet in equilibrium.13. Pentatomomorpha reveal gradual loss of embryo rotation, while Hydrocorisae retain such rotation, sometimes with germ-band and prolarval rotations. Both groups show a transition (anagenetic intra se, cladogenetic inter se ) from the immersed towards the superficial type of embryogenesis.14. The superficial condition of the hydrocorisous type prevails in Reduviidae after complete loss of rotations. The progression in embryonic evolution reached a high level in Harpactorinae; many perform semi-invagination, and species of Coranus entirely omit the invagination stage, and have no blastokinesis in the broadest sense. This deficiency is associated with early differentiation of the prospective germ band in the blastoderm stage. Similar early development occurs in some Hydrocorisae and, through cladistic divergence, also in evolved taxa of the Pentatomomorpha.15. The standard of embryogenesis is not influenced by egg shape. The dimensions of the embryo do not foreshadow those of the future larva but allometry of the limbs appears already during bud formation.16. In contrast to other Heteroptera, saldid embryos have the eyes differentiated before revolution. They possess a peculiar cephalic organ, possibly hydropic, extending through the serosal cuticle and underlying a great part of the chorion.17. A survey is given of the various positions and fates of the serosal hydropyle. The revolution of the embryo is predominantly brought about by local contractions of the fused amnion and serosa, and not by intrinsic action of the embryo itself.18. The remains of the contracted serosa, the serosal plug, is rapidly engorged in course of time into the future pronotal region to form the secondary dorsal organ. The involution is the result of a spectacular peristalsis, in the first instance caused by sudden contraction of cells close to the line of fusion between amnion and serosa.19. In many Miridae, the serosal plug, with or without yolk content, persists till eclosion of the egg, apparently absorbing water from the outside in order to stretch the serosal cuticle. The subsequent lengthening of the egg enables the prolarva to escape out of the sunken oviposition slit.20. The blackening of the egg is reducible to different principles, depending on whether suprachorionic, chorionic or subchorionic layers are involved. The blackish exudate of the serosal cuticle, restricted to some families of Heteroptera, may play a role in water regulation. Extra-embryonic envelopes of uncertain origin have been noticed in a few species.21. The embryological data are compared with the literature dealing with other insect Orders. Because the evolution of embryonic development of insects seems to be largely governed by parallelism, a clearer distinction between cladogenetic and anagenetic phenomena in other Orders must be made first before relationships between Orders can be settled. The differences between the holometabolic and the hemimetabolic type of development may not be as fundamental as has been suggested.22. The evolution of structures involved in four different methods of eclosion is outlined in fig. 278. A transverse, paired ruptor ovi forming part of the embryonic cuticle and delimiting the anteclypeus from the postclypeus is considered as the archetypical condition (as in Hebrus).23. Cladogenesis of the eclosion process evolved within the Amphibicorisae. In Mesovelia, eclosion is caused by fluid pressure within the embryonic cuticle. The situation in the Nabidae represents a link between this procedure and that in the Cimicoidea sensu lato, but the function of pressure transfer is gradually taken over by the fluid-filled serosal cuticle.24. The main device in eclosion of Reduviidae and Hydrocorisae is part of the serosal cuticle. Sudden forcing of extra-embryonic fluid anteriad explosively breaks the chorion in Hydrocorisae. Prolarval rotations may accelerate solution of the inner layer of the serosal cuticle, and the function of the pleuropodia is discussed in relation to this behaviour.25. In Amphibicorisae other than Mesoveliidae, and in the Leptopodoidea the transverse clypeal ruptor developed into a longitudinal frontal ruptor; Saldidae have both a frontal and a clypeal ruptor.26. Pentatomomorpha reveal anagenesis of the ruptor system resulting in displacement of the cephalic armature up to the pronotum through loss of the vertex.27. An account is given of the many sorts of bilateral, mostly monostrophous asymmetries found in the heteropterous egg system. The typical flexing pattern of limbs and antennae of the prolarva (a characteristic of the hemipteroid Orders) is racemic. In Hydrocorisae this asymmetry is constant and either amphidromous or monostrophous (the reverse asymmetry occurs in Plea ). 28. The different orientations of the laid egg are reduced to three main types (Table 2, p. 332) the evolution of which is outlined in fig. 281. Some speculations are made on the selection factors involved in switching from one type to the other.29. The archetypal Heteroptera did not possess a well developed ovipositor, and they were able both to deposit the eggs 'backwards' or 'forwards'. The theoretical possibilities of oviposition in ancestors are shown within the central circle of fig. 281.30. Saldidae and Mesoveliidae have a firm 'concave' ovipositor; oviposition is such that 180° rotation of the longitudinal axis of the egg within the genital tract must be supposed. Gerris regulates delivery of rotated or non-rotated eggs according to the oviposition site selected. Pentatomids of the genera Aeliomorpha and Macrina exhibit 90° rotation of the eggs and the alteration in egg shape conforms with this mode of laying.31. Comparison of embryogenic types (fig. 276), the egg types drawn in a standardized way (fig. 282-285), and the stance of the depositing female demonstrated that 180° rotation of the eggs is more common in Heteroptera and most probably also in other insects.32. The side of the egg where the embryonic anlage develops into the blastoderm is taken as the ventral side. The following rules have been drafted for the dorsoventral polarity of the egg-system: 1. All eggs laid exposed, whether rotated or not, and whose embryo rotates through 180°, are attached with the ventral side against the substrate; the venter of the fully grown embryo lies below the dorsal side of the egg. 2. Eggs without embryo rotation are likewise ventrally attached to the substrate, except when the egg is rotated before laying; the morphological sides of the fully grown embryo correspond with those of the egg.The same rules hold for erect or embedded eggs, when these are figured as being laid horizontally.33. The data obtained from the study of heteropterous eggs have been compared with relevant data from other Orders of insects. The 'HALLEZ law' is redefined on pages 347- 348.34. A preliminary discussion of the phylogeny of the Heteroptera is given. The group characters derived from the egg system are discussed on p. 350-363.35. The Leptopodoidea forms a natural group sharply defined from others. It seems improbable that Amphibicorisae arose from a proto-saldid stock; the opposite direction of evolution, Saldidae from proto-amphibicorisae is more in accordance with our findings.36. The Amphibicorisae appear more diverse than was assumed on the basis of other character complements, and comprise more than one superfamily. Mesoveliidae and Hydrometridae deviate considerably from the group type. Gerridae and Veliidae could be delimited more clearly from each other. Macrovelia and other aberrant genera show close affinity with Veliidae, not with Mesoveliidae.37. Pentatomomorpha constitute a natural group of families but the Idiostolidae are remote. The families are distinguished by the height reached on the anagenetic scale. The origin of the Pentatomoidea dated further back and the anagenesis advanced further than in the other superfamilies. Stenocephalidae appear more lygaeid-like and Colobathristidae more coreid-like. Malcidae are cladogenetically derivable from Piesmatidae. Eggs of Pseudophloeinae and Hydara resemble those of Alydidae.38. Reduvioidea, Thaumastocoroidea and Dipsocoroidea ought to be excluded from the Cimicomorpha which should contain only the families of the Cimicoidea sensu lato. On the basis of the eggs, Bryocorinae, excluding the Helopeltis group, merit family status. Several mirid genera seem to be classed under wrong subfamilies. The eggs of Velocipedidae and Pachynomidae are essentially nabid-like.39. Thaumastocoridae, Dipsocoroidea and Enicocephalidae are all isolated groups. There seems to be no justification for combining the two latter in one group.40. Hydrocorisae, inclusive of Corixidae and Ochteridae, share similar types of embryogenesis and eclosion dynamics but are heterogeneous in chorionic architecture. The common predecessors of Hydrocorisae probably must be found in the naucorid, not in the ochterid branch. Several taxa, considered as subfamilies, perhaps merit family rank (Potamocorinae, Aphelocheirinae, Diaprepocorinae, Micronectinae).41. The unintentional nomenclatoral consequences of the new major classification of terrestrial Heteroptera (LESTON et al. 1954) are discussed. Terrestrial Heteroptera are widely polyphyletic. Hence, the taxonomic use of the name Geocorisae should be avoided.42. The results of our study lead to the recognition of the following, more or less equivalent, major groups; Amphibicorisae, Leptopodoidea, Cimicomorpha sensu stricto, Dipsocoroidea, Enicocephaloidea, Reduvioidea, Thaumastocoroidea, Pentatomomorpha and Hydrocorisae (eggs of Joppeicidae were not available).43. Fig. 306 presents a provisional scheme of the phylogeny of the Suborder. The terrestrial groups and the Hydrocorisae are presented as radiations from an extinct amphibicorisous stock.44. The book concludes with a concise account of some new aspects of evolutionary morphology, which will be elaborated later. The subjects refer, among others, to: eye of egg larva, trichobothria, pretarsus, scent glands, internal and external genitalia, caryotypes, salivary glands, alimentary system, stigmata. These and other characters will be evaluated and compared in Parts 2 and 3 of the series in search of a basis for phylogenetic weighting