No limits: Breaking constraints in insect miniaturization

Miniaturization does not change conserved spider anatomy, a case study on spider Rayforstia (Araneae: Anapidae)

Miniaturization is an evolutionary trend observed in many animals. Some arachnid groups, such as spiders and mites, demonstrate a strong tendency toward miniaturization. Some of the most miniaturized spiders belong to the family Anapidae. In this study, using light and confocal microscopy and 3D modelling, we provide the first detailed description of the anatomy of a spider of the genus Rayforstia, which is only 900 µm long. In comparison with larger spiders, Rayforstia has no branching of the midgut in the prosoma and an increased relative brain volume. In contrast to many miniature insects and mites, the spider shows no reduction of whole organ systems, no allometry of the digestive and reproductive systems, and also no reduction of the set of muscles. Thus, miniature spider shows a more conserved anatomy than insects of a similar size. These findings expand our knowledge of miniaturization in terrestrial arthropods.

Anatomy of the miniature four-legged mite Achaetocoptes quercifolii (Arachnida: Acariformes: Eriophyoidea)

Miniaturization is one of the important trends in the evolution of terrestrial arthropods. In order to study adaptations to microscopic sizes, the anatomy of the smallest insects was previously studied, but not the anatomy of the smallest mites. Some of the smallest mites are Eriophyidae. In this study we describe for the first time the anatomy of the mite Achaetocoptes quercifolii, which is about 115 μm long. For this purpose, we used light, scanning, and transmission electron microscopy and performed 3D reconstructions. The anatomy of A. quercifolii is compared with the anatomy of larger representatives of Eriophyoidea. Despite the small size of the studied species, there is no considerable simplification of its anatomy compared to larger four-legged mites. A. quercifolii has a number of miniaturization effects similar to those found in microinsects: a strong increase in the relative volume of the reproductive system, an increase in the relative volume of the brain, reduction in the number and size of cells of the nervous system. As in some larger four-legged mites, A. quercifolii undergoes midgut lysis at the stage of egg production. On the other hand, in A. quercifolii a greater number of opisthosomal muscles are preserved than in larger gall-forming four-legged mites.

Breaking constraints: The development and evolution of extreme fin morphology in the Bramidae

The developmental process establishes the foundation upon which natural selection may act. In that same sense, it is inundated with numerous constraints that work to limit the directions in which a phenotype may respond to selective pressures. Extreme phenotypes have been used in the past to identify tradeoffs and constraints and may aid in recognizing how alterations to the Baupläne can influence the trajectories of lineages. The Bramidae, a family of Scombriformes consisting of 20 extant species, are unique in that five species greatly deviate from the stout, ovaloid bodies that typify the bramids. The Ptericlinae, or fanfishes, are instead characterized by relatively elongated body plans and extreme modifications to their medial fins. Here, we explore the development of Bramidae morphologies and examine them through a phylogenetic lens to investigate the concepts of developmental and evolutionary constraints. Contrary to our predictions that the fanfishes had been constrained by inherited properties of an ancestral state, we find that the fanfishes exhibit both increased rates of trait evolution and differ substantially from the other bramids in their developmental trajectories. Conversely, the remaining bramid genera differ little, both among one another and in comparison, to the sister family Caristiidae. In all, our data suggest that the fanfishes have broken constraints, thereby allowing them to mitigate trade‐offs on distinctive aspects of morphology.

Revision of the World Species of Megaphragma Timberlake (Hymenoptera: Trichogrammatidae)

Megaphragma species are important models for basic organismal research, and many are potential biological control agents. We present the first extensive revision of species of the genus Megaphragma based on morphological and molecular data. Our revision includes all previously described species, 6 of which are synonymized, and 22 of which are described here as new. We also provide the first key to all species of the genus and reconstruct their phylogeny based on 28S and CO1 molecular markers. The following species are synonymized with M. longiciliatum Subba Rao: M. aligarhensis Yousuf and Shafee syn. nov.; M. amalphitanum Viggiani syn. nov.; M. decochaetum Lin syn. nov.; M. magniclava Yousuf and Shafee syn. nov.; M. shimalianum Hayat syn. nov.M. anomalifuniculi Yuan and Lou syn. nov. is synonymized with M. polychaetum Lin. The following species are described as new: M. antecessor Polaszek and Fusu sp. nov.; M. breviclavum Polaszek and Fusu sp. nov.; M. chienleei Polaszek and Fusu sp. nov.; M. cockerilli Polaszek and Fusu sp. nov.; M. digitatum Polaszek and Fusu sp. nov.; M. fanenitrakely Polaszek and Fusu sp. nov.; M. funiculatum Fusu, Polaszek, and Viggiani sp. nov.; M. giraulti Viggiani, Fusu, and Polaszek sp. nov.; M. hansoni Polaszek, Fusu, and Viggiani sp. nov.; M. kinuthiae Polaszek, Fusu, and Viggiani sp. nov.; M. liui Polaszek and Fusu sp. nov.; M. momookherjeeae Polaszek and Fusu sp. nov.; M. nowickii Polaszek, Fusu, and Viggiani sp. nov.; M. noyesi Polaszek and Fusu sp. nov.; M. pintoi Viggiani sp. nov.; M. polilovi Polaszek, Fusu, and Viggiani sp. nov.; M. rivelloi Viggiani sp. nov.; M. tamoi Polaszek, Fusu, and Viggiani sp. nov.; M. tridens Fusu, and Polaszek sp. nov.; M. uniclavum Polaszek and Fusu sp. nov.; M. vanlentereni Polaszek and Fusu sp. nov.; M. viggianii Fusu, Polaszek, and Polilov sp. nov.

Structure of the Brain of the Smallest Coleoptera

The structure of the brain of the smallest coleopteran, Scydosella musawasensis Hall, 1999, is described for the first time. As in other extremely small beetles, the brain of S. musawasensis displays signs of miniaturization: displacement to the thorax, compactization, and a small number and size of the neurons. The body size of the studied smallest beetle is similar to that of the minute hymenopteran Megaphragma, which has a nearly anucleate nervous system. However, the structure of the brain of the studied smallest beetle is similar to that of large representatives of the order and is characterized by a high number of nuclei in the brain and a significant volume of the cell body rind. The neuropil of S. musawasensis occupies 60% of the brain volume, confirming the neuropilar constant rule.

On the Nature of Organs and Organ Systems – A Chapter in the History and Philosophy of Biology

Contrasting definitions of organs based either on function or on strictly morphological criteria are the legacy of a tradition starting with Aristotle. This floating characterization of organs in terms of both form and function extends also to organ systems. The first section of this review outlines the notions of organ and body part as defined, explicitly or implicitly, in representative works of nineteenth century’s comparative morphology. The lack of a clear distinction between the two notions led to problems in Owen’s approach to the comparative method (definition of homolog vs. nature of the vertebrate archetype) and to a paradoxical formulation, by Anton Dohrn, of the principle of functional change. Starting from the second half of the twentieth century, with the extensive use of morphological data in phylogenetic analyses, both terms – organ and body part – have been often set aside, to leave room for a comparison between variously characterized attributes (character states) of the taxa to be compared. Throughout the last two centuries, there have been also efforts to characterize organs or body parts in terms of the underlying developmental dynamics, both in the context of classical descriptive embryology and according to models suggested by developmental genetics. Functionally defined organ are occasionally co-extensive with morphologically defined body parts, nevertheless a clear distinction between the former and the latter is a necessary prerequisite to a study of their evolution: this issue is discussed here on the example of the evolution of hermaphroditism and gonad structure and function.

The reproductive system of Ceracis cornifer (Mellié) and first description of sperm structure in a minute tree-fungus beetle (Tenebrionoidea: Ciidae)

Studies on the spermatogenesis of Tenebrionidae beetles (Tenebrionoidea) have shown an unusual organization of spermatozoa, in which they are arranged antiparallelly within the testicular cysts. Despite such works, many taxa of Tenebrionoidea remain to be studied, including the minute tree-fungus beetles (Ciidae). Among the challenges in the study of the internal morphology of ciids is their small body size (about 2 mm or less), which makes dissections and comparisons extremely arduous. Here, we provide, for the first time, an anatomical and histological study of the reproductive system (female and male) and a description of sperm structure of Ceracis cornifer Mellié (Ciidae), under light and transmission electron microscopes. We pointed out the growth of a single oocyte at a time in females, aside of evidences toward a constant copulatory activity that can provide a continuous supply of sperm in their small spermatheca. In males, sperm have an antiparallel organization within the cysts, a condition observed so far only in members of Tenebrionoidea. Furthermore, we observed sperm dimorphism in C. cornifer. We described, for the first time, this characteristic in a Tenebrionoidea taxon, and discussed the possible relationship between sperm dimorphism and antiparallel organization of sperm.

Lizards as models to explore the ecological and neuroanatomical correlates of miniaturization

Extreme body size reductions bring about unorthodox anatomical arrangements and novel ways in which animals interact with the environment. Drawing from studies of vertebrates and invertebrates, we provide a theoretical framework for miniaturization to inform hypotheses using lizards as a study system. Through this approach, we demonstrate the repeated evolution of miniaturization across 11 families and a tendency for miniaturized species to occupy terrestrial microhabitats, possibly driven by physiological constraints. Differences in gross brain morphology between two gecko species demonstrate a proportionally larger telencephalon and smaller olfactory bulbs in the miniaturized species, though more data are needed to generalize this trend. Our study brings into light the potential contributions of miniaturized lizards to explain patterns of body size evolution and its impact on ecology and neuroanatomy. In addition, our findings reveal the need to study the natural history of miniaturized species, particularly in relation to their sensory and physiological ecology.

AC-DC electropenetrography: fundamentals, controversies, and perspectives for arthropod pest management

Studying the intimate association of arthropods with their physical substrate is both important and challenging. It is important because substrate is a key determinant for organism fitness; challenging because the intricacies of this association are dynamic, and difficult to record and resolve. The advent of electropenetrography (EPG) and subsequent developments allowed researchers to overcome this challenge. Nonetheless, EPG research has been historically restricted to piercing-sucking hemipteran plant pests. Recently, its potential use has been greatly broadened for additional pests with instrument advances. Thus, blood-feeding arthropods and chewing feeders, as well as non-feeding behaviors like oviposition by both pests and parasitoids, are novel new targets for EPG research, with critical consequences for integrated pest management. EPG can explain mechanisms of crop damage, plant or animal pathogen transmission, and the effects of insecticides, antifeedants, repellents, or transgenic plants and animals, on specific behaviors of damage or transmission. This review broadly covers the principles and development of EPG technology, emphasizing controversies and challenges remaining with suggested research to overcome them. In addition, it summarizes 60+ years of basic and applied EPG research, and previews future directions for pest management. The goal is to stimulate new applications for this unique enabling technology. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Miniaturization: How many cells are needed to build a tooth?

BACKGROUND

Organs that develop early in life, and are replaced by a larger version as the animal grows, often represent a miniature version of the adult organ. Teeth constituting the first functional dentition in small-sized teleost fish, such as medaka (Oryzias latipes), are examples of such miniature organs. With a dentin cone as small as the size of one human cell, or even smaller, these teeth raise the question how many dentin-producing cells (odontoblasts) are required to build such a tooth, and whether this number can be as little as one.

RESULTS

Based on detailed observations with transmission electron microscopy (TEM) and TEM-based 3D-reconstructions, we show that only one mesenchymal cell qualifies as a true odontoblast. A second mesenchymal cell potentially participates in dentin formation, but only at a late stage of tooth development. Moreover, the fate of these cells appears to be specified very early during tooth development.

CONCLUSIONS

Our observations indicate that in this system, one single odontoblast fulfills roles normally exerted by a large and communicating cell population. First-generation teeth in medaka thus provide an exciting model to study integration of multiple functions into a single cell.

Sensation of the tiniest kind: the antennal sensilla of the smallest free-living insect Scydosella musawasensis (Coleoptera: Ptiliidae)

Miniaturization is a major evolutionary trend prominent in insects, which has resulted in the existence of insects comparable in size to some unicellular protists. The adaptation of the complex antennal multisensory systems to extreme miniaturization is a fascinating problem, which remains almost unexplored. We studied the antennal sensilla of Scydosella musawasensis Hall, 1999 (Coleoptera: Ptiliidae), the smallest free-living insect, using scanning electron microscopy. The antenna of S. musawasensis bears 131 sensilla; no intraspecific variation in the number or position of the sensilla has been revealed. Nine different morphological types of sensilla are described according to their external morphological features and distribution: four types of sensilla trichodea, one type of sensilla chaetica, two types of sensilla styloconica, and two types of sensilla basiconica. Morphometric analysis of the sensilla of S. musawasensis, based on measurements of the lengths and diameters of sensilla and their location and number, showed the absence of significant differences between females and males. Comparative allometric analysis of S. musawasensis and larger Coleoptera showed that the number of sensilla and the size of sensilla chaetica decrease with decreasing body size. However, the number of the types of sensilla and the length and diameter of the multiporous sensilla basiconica revealed no correlation with the body size. Comparison of the acquired data with the results of our earlier study of the antennal sensilla of some of the smallest parasitic wasps is used to put forward hypotheses on the common principles of miniaturization of the antennal sensory systems of insects.

Effects of miniaturization in the anatomy of the minute springtail Mesaphorura sylvatica (Hexapoda: Collembola: Tullbergiidae)

Smaller animals display pecular characteristics related to their small body size, and miniaturization has recently been intensely studied in insects, but not in other arthropods. Collembola, or springtails, are abundant soil microarthropods and form one of the four basal groups of hexapods. Many of them are notably smaller than 1 mm long, which makes them a good model for studying miniaturization effects in arthropods. In this study we analyze for the first time the anatomy of the minute springtail Mesaphorura sylvatica (body length 400 µm). It is described using light and scanning electron microscopy and 3D computer reconstruction. Possible effects of miniaturization are revealed based on a comparative analysis of data from this study and from studies on the anatomy of larger collembolans. Despite the extremely small size of M. sylvatica, some organ systems, e.g., muscular and digestive, remain complex. On the other hand, the nervous system displays considerable changes. The brain has two pairs of apertures with three pairs of muscles running through them, and all ganglia are shifted posteriad by one segment. The relative volumes of the skeleton, brain, and musculature are smaller than those of most microinsects, while the relative volumes of other systems are greater than or the same as in most microinsects. Comparison of the effects of miniaturization in collembolans with those of insects has shown that most of the miniaturization-related features of M. sylvatica have also been found in microinsects (shift of the brain into the prothorax, absent heart, absence of midgut musculature, etc.), but also has revealed unique features (brain with two apertures and three pairs of muscles going through them), which have not been described before.