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.

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.

Structure and Ultrastructure of the Acrotrichis grandicollis (Coleoptera: Ptiliidae) Compound Eyes and the Eye Features Related to Miniaturisation.

Here is the first description of the visual organ of Acrotrichis grandicollis, a member of Ptiliidae family that includes the smallest free-living insects. The apposition eyes of A. grandicollis have the acone-type crystalline cone; a fused rhabdom is formed by eight retinula cells and surrounded by primary pigment cells. Secondary pigment cells are located only in the distal part of the ommatidium under the lens. The eye consists of about 50 facets about 12 µm in diameter. The A. grandicollis ommatidium ultrastructure has been compared with that of large Coleoptera. The results obtained enabled us to emphasize the compound eye specific features associated with the small body size.

[Hsp70 Genes of the Megaphragma amalphitanum (Hymenoptera: Trichogrammatidae) Parasitic Wasp].

Miniaturization is an evolutionary process that is widely represented in both invertebrates and vertebrates. Miniaturization frequently affects not only the size of the organism and its constituent cells, but also changes the genome structure and functioning. The structure of the main heat shock genes (hsp70 and hsp83) was studied in one of the smallest insects, the Megaphragma amalphitanum (Hymenoptera: Trichogrammatidae) parasitic wasp, which is comparable in size with unicellular organisms. An analysis of the sequenced genome has detected six genes that relate to the hsp70 family, some of which are apparently induced upon heat shock. Both induced and constitutively expressed hsp70 genes contain a large number of introns, which is not typical for the genes of this family. Moreover, none of the found genes form clusters, and they are all very heterogeneous (individual copies are only 75-85% identical), which indicates the absence of gene conversion, which provides the identity of genes of this family in Drosophila and other organisms. Two hsp83 genes, one of which contains an intron, have also been found in the M. amalphitanum genome.

Metagenomic analysis of microbial community of a parasitoid wasp Megaphragma amalphitanum.

The vast majority of multicellular organisms coexist with bacterial symbionts that may play various roles during their life cycle. Parasitoid wasp Megaphragma amalphitanum (Hymenoptera: Trichogrammatidae) belongs to the smallest known insects whose size is comparable with some bacteria. Using 16S rRNA gene sequencing and Whole Genome Sequencing (WGS), we described microbiota diversity for this arthropod and its potential impact on their lifecycle. Metagenomic sequences were deposited to SRA database which is available at NCBI with accession number SRX2363723 and SRX2363724. We found that small body size and limited lifespan do not lead to a significant reduction of bacterial symbionts diversity. At the same time, we show here a specific feature of microbiota composition in M. amalphitanum - the absence of the Rickettsiaceae family representatives that are known to cause sex-ratio distortion in arthropods and well represented in other populations of parasitoid wasps.