Revealing the larval anatomy of the hoverfly Sphaerophoria rueppellii (Wiedemann, 1820) (Diptera, Syrphidae) using micro-computed tomography

Micro-computed tomography (micro-CT) is an X-ray-based technique that allows visualisation of the internal anatomy of insects in situ and does not require dissections. Traditionally, the study of insect anatomy has been mainly based on dissection techniques and microtome sections. However, micro-CT is becoming an increasingly widespread study technique. We report the use of micro-CT scans to study, in detail, the external and internal structures and organs of the third instar larva of the dipteran Syrphidae Sphaerophoria rueppellii, a Palaearctic species included in the list of effective agents used in European biocontrol programmes against aphids. Detailed images and videos provided evidence of the external morphological characteristics and an overview of the internal anatomy (musculature, digestive tube, salivary glands, Malpighian tubules, fat body, circulatory system, nervous system, cephalopharyngeal complex, tentorium, and mouth parts). The study evidenced the following: an external lateroventral complex of muscles that form muscular rings that allow the compression of the body segments in a similar functional way as occurs in annelid worms; the existence of mandibles with prosthecae; the nervous system present a conspicuous distinguishable suboesophageal ganglion; the so-called antennomaxillary organs are the antennae; the maxillae are separate structures, with an articulated maxillary palpus; S. rueppellii has only one pair of Malpighian tubules instead of the two pairs that were thought to be the general rule for Syrphid larvae; and the evidence of an imaginal disc of genitalia in the posterior part of the body, below the rectum, which in male larvae has a clear correspondence with the morphology of the external male genitalia, which allows to differentiate the sexes in the larval stage. This study constitutes a true anatomical atlas of the third instar larvae of S. rueppellii. High-quality rendered images and additional supplementary videos together with a 3D model, suitable for use with mobile devices, are useful tools for future research and teaching aids.