Ultrastructural studies of sensilla in one fly of forensic importance

Taxonomic identification is essential in the field of forensic entomology. Insects are important in the decomposition of cadavers. The flies are generally attracted to cadavers and one of the most important contributions is to estimate the postmortem interval. The scanning electron microscopy (SEM), which allows rapid and accurate identification of different species of forensics flies, has been highlighted as it allows better visualization of the external morphology of immature and some adults. The purpose of this study was to examine in details the morphological aspects of sensilla on the antennae of female and male of Xanthacrona bipustulata Wulp using SEM, since many of them could not be observed just by the use of light microscopy.


EntomoBrasilis 13: e849 (2020)
he flies of the family Ulidiidae measure around 2 to 14 mm, usually with a dark and yellow coloration. There are 716 species differentiated in 113 genera (Kameneva et al. 2010;Tepedino et al. 2017). Most species of this family are saprophagous, but some species have developed the phytophagous habit. The larval and adult stages can feed on a variety of vegetables. Some adults present in this family are attracted by organic matter in the process of decomposition as leaves, fruits, trunks, carcasses and feces (Souza & LinhareS 1997;marchiori & SiLva 2001). The presence of Ulidiidae species in animal carcasses is rare, the exudates from decaying carcasses could serve as substrate for the development of the larvae, which are usually found in decaying plant material (moreTTi et al. 2008).
Ultrastructure studies using Scanning Electron Microscopy (SEM) have been extensively conducted to clarify some structures on Hexapoda: LopeS et al. (2002) found two types of basiconic sensilla on antennae of Phoracantha semipunctata (Fabricius) (Coleoptera); SuKonTaSon et al. An ultrastructure analysis of the morphological aspects of sensilla on the antennae of X. bipustulata have not been performed yet. The purpose of this study was to examine in details these sensorial organs on the aforementioned species to help increase the anatomical database.
The flie X. bipustulata was obtained from one carrion of Oryctolagus sp and the collections were performed daily, during january of 2018, in an Atlantic rainforest fragment at "Instituto Samambaia de Ciências Ambientais e Ecoturismo" (ISCA) -Petrópolis, Rio de Janeiro, Brazil (22º46'90" S and
Specimens were properly identified following Brown et al. (2009). The terminologies of adult's flies used in this study followed meLLo (2003)  The heads were dissected from one male and five females under dissecting microscope. Then, these structures were processed for SEM examination by transferring to 2.5% of glutaraldehyde mixture in phosphate buffered saline (PBS) for 24 h. Afterwards, specimens were rinsed twice with PBS during 10-minute-intervals and post fixed with 1% of osmium tetroxide at room temperature for 3 days. This post fixation step was carried out under a well-ventilated fume hood. The flies were then rinsed twice with PBS and dehydrated in an increasing graded series of ethanol (30, 50, 70, 80 and 90%) for 12 h during each step. Thereafter, flies were placed in absolute alcohol, followed by a treatment in absolute acetone. On the next step, the specimens were subjected to critical point drying and they were later placed on metallic supports, coated with a thin gold layer (20-30 nm) and examined under JEOL 6390LV scanning electron microscope (SEM) (Akishima, Tokyo, Japan).

RESULTS
SEM observations of the head of male and female of X. bipustulata revealed a pair of antennae, frontally situated, between the large compound's eyes (Figure 1). Antenna morphology consisting in three segments: the first called scape (Sc), the second pedicel (Pe) and a third flagellum observed in female composed of a seta called arista (Ar) located laterally and an enlarged basal flagellomere (Fn) (Figure 1). All antennal segments in this species have sensilla.
Male: The scape is densely covered by microtrichia, and only one type of sensilla was observed: eight-sensilla chaetica (Ch I) of similar length and organized in a single row (Figure 2A). The surface of the pedicel is covered by microtrichia similar to those found in the scape and presents two types of chaetica sensilla (Ch I and ChII). The first type (Ch I) is represented by 14 bristles, similar to those found in the scape. The second type (Ch II) is represented by a single bristle located in the distal region ( Figure 3A).
Female: The scape is densely covered by microtrichia, and only one type of sensilla was observed: eight-sensilla chaetica (Ch I) of similar length and arranged in a single row ( Figure 2B). The surface of the pedicel is covered by microtrichia similar to those found in the scape and presents two types of chaetica sensilla (Ch I and ChII). The first type (Ch I) is represented by 15 bristles, similar to those found in the scape. The second type (Ch II) is represented by a single bristle located in the distal region (Figure 3 B). The flagellum is the largest segment of the antenna and numerous sensilla are found in this segment. Arising closed to the base of the first flagellomere on its dorsolateral surface is the arista, which is composed of two segments (I-II) ( Figure 4A). The Fn is densely covered by microtrichia similar to those found in the scape and pedicel. Among the microtrichia has a type of sensilla: trichoidea (tr) ( Figure 4B).

DISCUSSION
The ultrastructure of the antennae of Ulidiidae species using SEM are limited in the literature, theses sensorial structure have been performed, such as: SuKonTaSon et al. The external morphology of the antennae segments of X. bipustulata is generally similar to that of other Ulidiidae and calyptrate flies (SuKonTaSon et al. 2006;carriço et al. 2015;pezzi et al. 2016;caeTano et al. 2018). In this study, the microtrichia morphology of the antennae is similar to that of other calyptrate and are present on all antennal surface with a variable distribution (SuKonTaSon et al. 2004;SuKonTaSon et al. 2007;SeTzu et al. 2011;zhanG et al. 2013a;zhanG et al. 2013b;carriço et al. 2015;pezzi et al. 2016;caeTano et al. 2018).

B A B A
number of sensory sensilla can vary from species to species (SuKonTaSon et al. 2004;carriço et al. 2015). One types of sensilla on female was observed: trichoidea (tr); those distribution differ of other dipteran species, such as O. chalcogaster (SuKonTaSon et al., 2007) In conclusion, taxonomical studies are important to the biological sciences in general, because erroneous species identifications can mislead expert reports, additionally, this investigation provided new findings of some diagnostic structures of flies using SEM, since many of them are not possibly observed just by the use of light microscopy