Endopterygota

Abstract: Development of the insect spermatozoon - spermatogenesis the fertilizing spermatozoon phylogeny of the hexapod orders superclass hexapoda class insecta (ectognathous hexapods) subclass apterygota subclass pterygota (introduction) infraclass palaeopter infraclass neoptera orthoptera and phasmatodea orders embioptera, dermapter, plecoptera and grylloblattodea the hemipteroid (rhynchotoid) orders the hemipteroid orders - hemiptera suborder heteroptera the endopterygota (holometabola) order coleoptera orders mecoptera and siphonaptera orders diptera and strepsiptera superorder amphiesmenoptera order hymenoptera taxonomic summary and phylogenetic analysis.

Abstract: In contemporary entomology the morphological characters of insects are not always treated according to their phylogenetic rank. Fossil evidence often gives clues for different interpretations. All primitive Paleozoic pterygote nymphs are now known to have had articulated, freely movable wings reinforced by tubular veins. This suggests that the wings of early Pterygota were engaged in flapping movements, that the immobilized, fixed, veinless wing pads of Recent nymphs have resulted from a later adaptation affecting only juveniles, and that the paranotal theory of wing origin is not valid. The wings of Paleozoic nymphs were curved backwards in Paleoptera and were flexed backwards at will in Neoptera, in both to reduce resistance during forward movement. Therefore, the fixed oblique-backwards position of wing pads in all modern nymphs is secondary and is not homologous in Paleoptera and Neoptera. Primitive Paleozoic nymphs had articulated and movable prothoracic wings which became in some modern insects transformed into prothoracic lobes and shields. The nine pairs of abdominal gillplates of Paleozoic mayfly nymphs have a venation pattern, position, and development comparable to that in thoracic wings, to which they are serially homologous. Vestigial equivalents of wings and legs were present in the abdomen of all primitive Paleoptera and primitive Neoptera. The ontogenetic development of Paleozoic nymphs was confluent, with many nymphal and subimaginal instars, and the metamorphic instar was missing. The metamorphic instar originated by the merging together of several instars of old nymphs; it occurred in most orders only after the Paleozoic, separately and in parallel in all modern major lineages (at least twice in Paleoptera, in Ephemeroptera and Odonata; separately in hemipteroid, blattoid, orthopteroid, and plecopteroid lineages of exopterygote Neoptera; and once only in Endopterygota). Endopterygota evolved from ametabolous, not from hemimetabolous, exopterygote Neoptera. The full primitive wing venation consists of six symmetrical pairs of veins; in each pair, the first branch is always convex and the second always concave; therefore costa, subcosta, radius, media, cubitus, and anal are all primitively composed of two separate branches. Each pair arises from a single veinal base formed from a sclerotized blood sinus. In the most primitive wings the circulatory system was as follows: the costa did not encircle the wing, the axillary cord was missing, and the blood pulsed in and out of each of the six primary, convex-concave vein pair systems through the six basal blood sinuses. This type of circulation is found as an archaic feature in modern mayflies. Wing corrugation first appeared in preflight wings, and hence is considered primitive for early (paleopterous) Pterygota. Somewhat leveled corrugation of the central wing veins is primitive for Neoptera. Leveled corrugation in some modern Ephemeroptera, as well as accentuated corrugation in higher Neoptera, are both derived characters. The wing tracheation of Recent Ephemeroptera is not fully homologous to that of other insects and represents a more primitive, segmental stage of tracheal system. Morphology of an ancient articular region in Palaeodictyoptera shows that the primitive pterygote wing hinge in its simplest form was straight and composed of two separate but adjoining morphological units: the tergal, formed by the tegula and axillaries; and the alar, formed by six sclerotized blood sinuses, the basivenales. The tergal sclerites were derived from the tergum as follows: the lateral part of the tergum became incised into five lobes; the prealare, suralare, median lobe, postmedian lobe and posterior notal wing process. From the tips of these lobes, five slanted tergal sclerites separated along the deep paranotal sulcus: the tegula, first axillary, second axillary, median sclerite, and third axillary. Primitively, all pteralia were arranged in two parallel series on both sides of the hinge. In Paleoptera, the series stayed more or less straight; in Neoptera, the series became V-shaped. Pteralia in Paleoptera and Neoptera have been homologized on the basis of the fossil record. A differential diagnosis between Paleoptera and Neoptera is given. Fossil evidence indicates that the major steps in evolution, which led to the origin first of Pterygota, then of Neoptera and Endopterygota, were triggered by the origin and the diversification of flight apparatus. It is believed here that all above mentioned major events in pterygote evolution occurred first in the immature stages.

Abstract: As insects belonging to the Endopterygota (or Holometabola), the life cycle of Chironomidae is divided into four distinct stages, ie egg, larva, pupa and imago Notwithstanding the large number of species within the family, chironomids share one conspicuous life history characteristic in that the last two stages are generally very short in duration, while the lengths of egg and larval stages vary substantially between and within species This seems to be related to the fact that, as in some other aquatic insect orders such as the Ephemeroptera and Plecoptera, chironomid adults mostly rely upon the energy stored during the larval stage to accomplish reproduction, the single most important task assigned to them Though feeding is known to occur in adults (Chapter 9), this constitutes a tiny proportion of energy acquisition and does not contribute significantly to overall reproductive output, a situation which is clearly different from that of many other related dipterans such as blood-sucking blackflies and mosquitoes Clear demarcation of energy acquisition and reproduction along the phases of metamorphosis implies that the adult, reproductive phase is heavily dependent on the preceding feeding phase If a fixed amount of energy is to be allocated to the maintenance of the adult body and gamete production, the highest reproductive output would be achieved by minimizing the length of the adult stage, the strategy that seems to have been adopted by the vast majority of chironomid species