Understanding the evolution of the developmental programs active during dipteran embryogenesis depends on comparative studies. As a counterpoint to the intensively investigated and highly derived cyclorrhaphan flies that include the model organism Drosophila melanogaster, we are studying the basal Diptera Bradysia hygida, a member of the Sciaridae family that is amenable to laboratory cultivation. Here we describe the B. hygida embryogenesis, which lasts 9 days at 22 °C. The use of standard fixation D. melanogaster protocols resulted in embryos refractory to DAPI staining and to overcome this, a new enzyme-based method was developed. Calcofluor-White staining of enzimatically-treated embryos revealed that this method removes chitin from the serosal cuticle surrounding the B. hygida embryo. Chitin is one of the main components of serosal cuticles and searches in a B. hygida embryonic transcriptome database revealed conservation of the chitin synthesis pathway, further supporting the occurrence of chitin biosynthesis in B. hygida embryos. Combining the enzymatic treatment protocol with the use of both DIC and fluorescence microscopy allowed the first complete description of the B. hygida embryogenesis. Our results constitute an important step towards the understanding of early development of a basal Diptera and pave the way for future evo-devo studies.

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Sciarids, also called "fungus gnats" are small, almost entirely dark-coloured insects.Sciarid larvae feed on different substrates and can infest agricultural crops and mushroom nurseries, causing economic losses. Of the 2174 Diptera mitogenome sequences currently available in GenBank, only eight are from the Sciaridae family, none of which are complete circular molecules. Here we describe the mitogenome sequences of three sciarid species: Phytosciara flavipes, Trichosia splendens and Bradysia hygida and provide novel insights on the control region of sciarid mitogenomes. The assembled mitogenomes range from 16,062 bp in P. flavipes to 17,095 bp in B. hygida. All 13 protein coding genes, 22 tRNAs and 2 rRNAs characteristic of insect mitogenomes were identified, but the sequence of the control region could not be determined. Experimental results suggest that the B. hygida control region is about 21 kb long resulting in a 37 kb long mitogenome which constitutes the largest insect mitochondrial genome described so far. Phylogenetic analysis using all Bibionomorpha mitogenome sequences available in GenBank strongly supports the Sciaridae monophyly and led to the identification of species and subfamily specific gene rearrangements. Our study extends the knowledge of this large and diverse insect family that includes agricultural pest species.