Alexey A. Polilov

The smallest insects evolve anucleate neurons

The smallest insects are comparable in size to unicellular organisms. Thus, their size affects their structure not only at the organ level, but also at the cellular level. Here we report the first finding of animals with an almost entirely anucleate nervous system. Adults of the smallest flying insects of the parasitic wasp genus Megaphragma (Hymenoptera: Trichogrammatidae) have only 339-372 nuclei in the central nervous system, i.e., their ganglia, including the brain, consist almost exclusively of processes of neurons. In contrast, their pupae have ganglia more typical of other insects, with about 7400 nuclei in the central nervous system. During the final phases of pupal development, most neuronal cell bodies lyse. As adults, these insects have many fewer nucleated neurons, a small number of cell bodies in different stages of lysis, and about 7000 anucleate cells. Although most neurons lack nuclei, these insects exhibit many important behaviors, including flight and searching for hosts.

Miniaturisation effects in larvae and adults of Mikado sp. (Coleoptera: Ptiliidae), one of the smallest free-living insects

We present the first morphological study of larvae and adults of Mikado sp. - one of the smallest known beetles and free-living insects (body length of adult is 390-455microm). Morphological and developmental consequences of miniaturisation in Mikado and insects in general are discussed. We used histological sectioning, scanning electron microscopy, laser confocal microscopy and 3D-computer reconstruction. For the first time we report that according to the morphometric data of Mikado sp., at least some ptiliid beetles have three larval stages. We studied the muscular system of adults and larval stages. It is shown that ptiliid beetles have nearly the complete set of muscles found in larger staphyliniform beetles. Developmental and size dependent changes in the relative volume of different organs are addressed. All organ systems change allometrically in the development of Mikado sp. as well as in comparison with larger representatives of Ptiliidae and closely related groups of beetles, such as Staphylinidae. We conclude that the factors limiting miniaturisation are the size of the neural system, associated with the number and size of neurons, the mass of the skeleton, the egg size (free-living insects), and consequently the volume of the reproductive system.

Small Is Beautiful: Features of the Smallest Insects and Limits to Miniaturization

Miniaturization leads to considerable reorganization of structures in insects, affecting almost all organs and tissues. In the smallest insects, comparable in size to unicellular organisms, modifications arise not only at the level of organs, but also at the cellular level. Miniaturization is accompanied by allometric changes in many organ systems. The consequences of miniaturization displayed by different insect taxa include both common and unique changes. Because the smallest insects are among the smallest metazoans and have the most complex organization among organisms of the same size, their peculiar structural features and the factors that limit their miniaturization are of considerable theoretical interest to general biology.

Developmental stages of the hooded beetle Sericoderus lateralis (Coleoptera: Corylophidae) with comments on the phylogenetic position and effects of miniaturization.

The first detailed morphological study of larvae, pupae and adults of a species of the hooded beetles (Coleoptera: Corylophidae) -Sericoderus lateralis - is presented. Histological sectioning, scanning and transmission electron microscopy, laser confocal microscopy and 3D-computer reconstruction were used. For the first time we report that according to the morphometric data of S. lateralis, at least some corylophid beetles have three larval stages. A phylogenetic position of Corylophidae within a cucujoid-cleroid clade is confirmed, and also the placement of Sericoderini within a corlyophid subgroup, which does not include Periptycinae and Foadiini. The larvae of Sericoderus are mainly characterized by plesiomorphic features compared to those of other corylophid tribes, notably Peltinodini and Rypobiini. Morphological and developmental consequences of miniaturization are discussed. Corylophid beetles display much less specific and far-reaching morphological consequences of miniaturization compared to Ptiliidae. We report the presence of unique modifications in the neural system not shared with any other insects, such as a distinctly asymmetric supraoesophageal ganglion in first instar larva, and a total displacement of the brain to the thorax in the adult stage. A highly unusual feature of the digestive tract is the sclerotised, V-shaped ventral wall of the pharynx. Developmental and size dependent changes in the relative volume of different organs are addressed. All organ systems change allometrically in the development of S. lateralis. Allometric trends in the volume of organs confirm that the factors limiting miniaturization are the size of the neural system, associated with the number and size of neurons (most critical for first instar larva), the mass of the skeleton, the egg size, and consequently the volume of the reproductive system (for free-living insects).

Anatomy of the smallest coleoptera, featherwing beetles of the tribe nanosellini (Coleoptera, Ptiliidae), and limits of insect miniaturization

The internal structure of Nanosella sp., Primorskella sp., and Porophila sp. was described from serial sections and total preparations using light and transmission electron microscopy. The most important structural features related to miniaturization are the absence of midgut muscles, abortion of two Malpighian tubes, a decrease in the number of abdominal spiracles, strong reduction of the tracheal system, the absence of the heart, reduction of the circulatory system and its substitution by the fat body, strong oligomerization and concentration of the nervous system, a decrease in the size and number of neurons, and reduction of the left testis and left ovary. The internal structure of featherwing beetles was analyzed for the first time using 3D computer models, which allowed us to demonstrate changes in the relative volume of organs accompanying the body diminution. The excretory and digestive systems change isometrically, while others change allometrically. The relative volume of the musculature decreases, and that of the nervous and reproductive systems increases. The skeleton mass also increases. The possible factors limiting further diminution in Ptiliidae (egg size, the volume of the reproductive and nervous systems, and the skeleton mass) are discussed.

Unique rostrate larvae and basidiomycophagy in the beetle family Corylophidae.

The head morphology of larvae of two undescribed species of the corylophid genus Holopsis were examined. Both are associated with the same basidiomycete host Ganoderma cf applanatum. Whereas the round and convex adults are very similar, one of the disc-shaped larvae is characterized by an elongate weevil-like snout, which is a unique feature in larval beetles. The posterior head region, the mouthparts and the general configuration of the musculature are similar in the larvae of both species. However, in the rostrate Holopsis sp. 1 most muscles are either widened along the longitudinal axis or elongated. Moreover, an additional bundle of M. frontobuccalis posterior is present, which strengthens the pharyngeal pumping apparatus. Both species share an unusual connection between the prepharynx and pharynx. This is a potential autapomorphy of the genus. The larval cephalic morphology of Holopsis sp. 2 and the corylophine genus Sericoderus is quite similar. However, they differ in some muscular features and in the configuration of the foregut. Holopsis species are associated with Basidiomycetes. Whether this is an ancestral condition in Corylophidae remains ambiguous due to conflicting phylogenetic hypotheses and the largely unknown biology of the Australian subfamily Periptyctinae. Several features of Holopsis are likely plesiomorphic and possibly related with the association with basidiomycetes. However, the larval rostrum of sp. 1 is doubtlessly derived, and could have a performance advantage over other species feeding on the spores of Ganoderma cf applanatum including Holopsis sp. 2.

Peculiarities of the brain organization and fine structure in small insects related to miniaturization. 1. The smallest Coleoptera (Ptiliidae)

This paper describes for the first time the organization and fine structure of the brain in the smallest free-living insects Acrotrichis grandicollis, Micado sp., and Nanosella sp. (Ptiliidae, Coleoptera), which were studied using serial histological sections as well as TEM and computer-assisted 3D reconstructions. Some specific structural features related to miniaturization were revealed; the relative size of the brain regions and localization of its structures were analyzed. In spite of the extremely small body size, the brain retains the structure and fine structure typical of larger representatives of related groups, illustrating high conservatism of the brain morphology. Data on the number and size of neurons in the brain of Ptiliidae were obtained. The results obtained confirm and supplement the hypothesis about the factors limiting miniaturization of insects. [The next papers will describe the brain organization in Mymaridae and Trichogrammatidae (Hymenoptera), Corylophidae (Coleoptera), Thripidae (Thysanoptera), and Liposcelidae (Psocoptera).]

Features of the structure of hymenoptera associated with miniaturization: 1. Anatomy of the fairyfly Anaphes flavipes (Hymenoptera, Mymaridae)

Anatomy of the adult fairyfly Anaphes flavipes (Hymenoptera, Mymaridae) is described in detail for the first time, based on series of sections and 3D computer reconstruction. Despite its tiny size, A. flavipes does not demonstrate strong morphological simplification, except for the merging of certain skeletal elements, a slight simplification of endoskeleton, lack of midgut muscles, a reduced number of Malpighian tubules, simplification of the tracheal and circulatory systems, asymmetry of the central nervous system, and the absence of several muscles.

Features of the structure of hymenoptera associated with miniaturization: 2. Anatomy of Trichogramma evanescens (Hymenoptera, Trichogrammatidae)

Anatomy of adults of Trichogramma evanescens (Hymenoptera: Trichogrammatidae) is described in detail for the first time based on series of sections and 3D computer reconstruction. The complex structure of the exoskeleton and musculature (except for reductions of individual muscles) is preserved in T. evanescens despite its tiny size. Considerable simplification is observed in the structure of the intestine, tracheal and circulatory systems, and in the reduced number of Malpighian tubules. Thus, the smallest Hymenoptera (Mymaridae and Trichogrammatidae) demonstrate both the effects of miniaturization universal for insects (reduction of the head endoskeleton, circulatory and respiratory systems) and specific ones.

The anatomy of the thrips Heliothrips haemorrhoidalis (Thysanoptera, Thripidae) and its specific features caused by miniaturization

A new set of data on the internal and external structure of the adult and larva of the thrips Heliothrips haemorrhoidalis (Bouché, 1833) is presented. The structure of the internal systems of this thrips was revealed using modern methods of 3D computer modelling. The changes in shape and relative size are discussed as an outcome of miniaturization in comparison to the supposed ancestor of this species. The layout of the internal systems of thrips is compared to those of other insects similar in size: beetles of the families Ptiliidae and Corylophidae and wasps of the families Mymaridae and Trichogrammatidae.