Anatomy of adult Megaphragma (Hymenoptera: Trichogrammatidae), one of the smallest insects, and new insight into insect miniaturization

Abstract

The body size, especially in cases of extreme reduction, is an important characteristic that strongly determines the morphology, physiology, and biology of animals. Miniaturization is a widespread trend in animal evolution and one of the principal directions of evolution in insects. Miniaturization-related features of insect morphology have been subject to intensive studies during the last few years, but the structure of the smallest insects remains insufficiently known. It is especially important to study hymenopterans of the genus Megaphragma, which include the smallest flying insects and a species in which an almost anucleate nervous system was recently discovered. This article is the first detailed study of the external and internal morphology of adults of Megaphragma mymaripenne and M. amalphitanum using histological methods, 3D computer modeling and other techniques. It is shown that in spite of the extremely small size the organization of Megaphragma retains a considerkable level of structural complexity. On the other hand, miniaturization leads to re-organizations of several organ systems. Unique structural features related to miniaturization have been found in both species: lysis of cell bodies and nuclei of neurons at late stages of pupal development, absence of the heart, and considerable reductions in the set of muscles. Comparative analysis of structure in the smallest insects representing different taxa has revealed common features of the evolutionary process of miniaturization in insects.

First record of Megaphragma (Hymenoptera, Trichogrammatidae) in Columbia, and third animal species known to have anucleate neurons

“…Taking into account the relative volume and structure of the neuropil and the number of nuclei, which is similar to other species of Megaphragma examined earlier but fundamentally different from that of other minute insects. I suggest that M. caribea also displays the unique phenomenon of lysis of cell bodies and nuclei in neurons prior to the emergence of the adult from the pupa, as described earlier in M. mymaripenne (Polilov 2012) and M. amalphitanum (Polilov 2017). Thus, all species of Megaphragma whose anatomy has been studied have unique anucleate neurons.…”

Cephalic anatomy of Sphaeriusidae and a morphology‐based phylogeny of the suborder Myxophaga (Coleoptera)

“…In contrast, it is strongly simplified in some other miniaturized coleopterans: the dorsal arm is missing in many ptiliids (Polilov, ; Yavorskaya et al ., ) and both the dorsal and anterior arms are absent in some examined species of Corylophidae (Polilov & Beutel, ; Yavorskaya & Polilov, ). Simplifications of the tentorium were also observed in miniaturized adult hymenopterans (Polilov, ). It is completely reduced in corylophid species of the genus Orthoperus Stephens, 1829, in the moderately small archostematan species Micromalthus debilis (Micromalthidae) ( c .…”

A draft genome sequence of the miniature parasitoid wasp, Megaphragma amalphitanum

“…The wasp spends most of its life cycle in host eggs, while the imago stage is very short and lasts only a few days [3,4]. M. amalphitanum belongs to chalcid wasps, which represent one of the largest insect superfamilies (∼23,000 described species) [5]. The higher-level taxonomic relationships of Trichogrammatidae, Chalcidoidea and Hymenoptera have been investigated in several recent studies [6][7][8][9][10] that helped to establish the placement of this unique taxon that related to Mymaridae and Pteromalidae.…”

“…The higher-level taxonomic relationships of Trichogrammatidae, Chalcidoidea and Hymenoptera have been investigated in several recent studies [6][7][8][9][10] that helped to establish the placement of this unique taxon that related to Mymaridae and Pteromalidae. amalphitanum (image adapted from [5]), (B) T. namibiensis -the largest known bacterium (modified from Schulz et al 1999) [11].…”