Major differences in the larval anatomy of the digestive and excretory systems of three Oestridae species revealed by micro-CT

Comparative larval anatomy of the digestive system of three Calliphoridae (Diptera) species that cause different types of myiasis

The Calliphoridae are one of the main Diptera families that include agents of the parasitic disease condition known as myiasis. Parasitism seems to have evolved multiple independent times within the Calliphoridae; consequently, this family includes a diversity of myiasis-causing species, varying in their obligate or facultative habits and in their specific location in the host. Larval morphological studies can provide novel and relevant insights into the biology of those species, as well as on the pathogenesis and evolution of myiasis; however, the anatomy of internal organs and structures -particularly those within the digestive system- has generally been overlooked, despite potentially reflecting parasitic adaptations. We use here non-invasive X-ray micro-computed tomographic techniques to study the anatomy of the digestive system of the third instar larvae of three Calliphoridae species: Protocalliphora azurea, an obligate agent of sanguinivorous myiasis in passerine bird nestlings; Cordylobia anthropophaga, an obligate agent of subcutaneous myiasis in mammals; and Lucilia sericata, a facultative agent of traumatic myiasis in mammals. The three species are relatively uniform in the internal anatomy of their digestive organs, although they differ in the shape and size of the salivary glands -a main source of larval antigens-, which are considerably smaller in P. azurea. Moreover, the three species differ from the larvae of Oestridae, a close family that exclusively includes obligate myiasis-causing species, in the presence of gastric caeca and a crop, which shows a remarkable storage capacity in L. sericata. The observed differences are discussed from a functional perspective and in relation to the type of myiasis caused.

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.

Larval anatomy of the digestive and excretory systems of the pharyngeal bot fly, Pharyngomyia picta (Diptera: Oestridae)

Oestrid flies (Diptera: Oestridae) are obligate parasites of mammals during their larval stage and show anatomical adaptations for the infestation of host tissues. Unlike the species that parasitize domestic mammals, those oestrid species that infest wild mammal hosts remain poorly known. With the use of x-ray micro-computed tomography, we describe for the first time the anatomy of the digestive and excretory systems of the second and third larval instars of Pharyngomyia picta (Meigen), a parasite of cervids that, like other species within the subfamily Oestrinae, causes nasopharyngeal myiasis. Both larval instars of P. picta show a pair of remarkably large salivary glands arranged in a characteristic 'glandular band', a convoluted and thickly uniform midgut and a greatly enlarged distal region of the anterior pair of Malpighian tubules. These anatomical features also have been described in other species within the subfamily Oestrinae, whereas they differ from the observations in other oestrid subfamilies. We discuss the potential functional significance of the anatomy of the digestive and excretory systems of Oestrinae larvae as specific adaptations to parasitize the nasopharyngeal cavities of mammal hosts.

A quantitative micro-tomographic gut atlas of the lepidopteran model insect Manduca sexta

The tobacco hornworm is used extensively as a model system for ecotoxicology, immunology and gut physiology. Here, we established a micro-computed tomography approach based on the oral application of the clinical contrast agent iodixanol, allowing for a high-resolution quantitative analysis of the Manduca sexta gut. This technique permitted the identification of previously unknown and understudied structures, such as the crop or gastric ceca, and revealed the underlying complexity of the hindgut folding pattern, which is involved in fecal pellet formation. The acquired data enabled the volume rendering of all gut parts, the reliable calculation of their volumes, and the virtual endoscopy of the entire alimentary tract. It can provide information for accurate orientation in histology uses, enable quantitative anatomical phenotyping in three dimensions, and allow the calculation of locally effective midgut concentrations of applied chemicals. This atlas will provide critical insights into the evolution of the alimentary tract in lepidopterans.

When the Utility of Micro-Computed Tomography Collides with Insect Sample Preparation: An Entomologist User Guide to Solve Post-Processing Issues and Achieve Optimal 3D Models

Many techniques are used today to study insect morphology, including light and electron microscopy. Most of them require to specifically prepare the sample, precluding its use for further investigation. In contrast, micro-CT allows a sample to be studied in a non-destructive and rapid process, even without specific treatments that might hinder the use of rare and hard-to-find species in nature. We used synchrotron radiation (SR) micro-CT and conventional micro-CT to prepare 3D reconstructions of Diptera, Coleoptera, and Hymenoptera species that had been processed with 4 common preparation procedures: critical-point drying, sputter-coating, resin embedding, and air-drying. Our results showed that it is possible to further utilize insect samples prepared with the aforementioned preparation techniques for the creation of 3D models. Specimens dried at the critical point showed the best results, allowing us to faithfully reconstruct both their external surface and their internal structures, while sputter-coated insects were the most troublesome for the 3D reconstruction procedure. Air-dried specimens were suitable for external morphological analyses, while anatomical investigation of soft internal organs was not possible due to their shrinking and collapsing. The sample included in resin allowed us to reconstruct and appreciate the external cuticle and the internal parts. In this work, we demonstrate that insect samples destined to different analyses can be used for new micro-CT studies, further deepening the possibility of state-of-the-art morphological analyses.

Oestrid myiasis at a cross-road

The oestrids are a genuinely unique group of Diptera that represent many species whose larvae parasitize mammals, feeding on live tissues of a wide range of hosts, from mice to elephants, causing obligatory myiases. Those have had greatest impact on the activities of humans and their use of animals (e.g., as food, recreation, in the conduct of wars). To a large extent their impact has been controlled through the use of therapeutic application of synthetic chemical treatments. While in some limited cases eradication has been achieved, it is unlikely to become the norm as these parasites seem well adapted to survive at low population densities and small smoldering foci are likely to present a challenge in the future. Should organic production eschew the use of parasiticides, other than for rescue of seriously ill animals, the foci of infection by these parasites will remain. In this article, we explore current scientific information on this group of parasites that may have become an anachronism in modern livestock parasitology, identify gaps in our knowledge as well as the suggestions for the management of these parasites in the era of genomics and proteomics.

An Update on Cephenemyiosis in the European Roe Deer: Emergent Myiasis in Spain

Cephenemyia stimulator is a Palearctic species developing in the nasal cavity and pharynx of roe deer (Capreolus capreolus). It is widely spread in the range of distribution of this ungulate in Europe. Since the first report of C. stimulator in Spain in 2001, a rapid geographic expansion has been observed, first in the north of the country, with high prevalence and intensities of infestation that caused some mortal cases, and, lately, also in Extremadura and Andalucía, the southernmost populations of European roe deer. These observations suggest an adaptation of this parasite to different ecosystems of the Iberian Peninsula. Almost simultaneously, C. stimulator is also expanding its range to northern Europe, with the first cases being reported in Sweden. Thus, Cephenemyia stimulator may be an example of a parasite currently displaying distributional changes along its southernmost and northernmost range margins. Thus, it is of the utmost importance to unravel all the epidemiological and clinical aspects of this myiasis, as well as implementing surveillance measures including reliable and non-invasive diagnostic techniques to monitor its expansion and adaptation to different ecosystems and/or hosts and to reduce the negative impact on roe deer populations.

An X-ray micro-computer tomography study of the Malpighian tubules of the Blue Bottle Blow Fly (Calliphora vomitoria) Diptera: Calliphoridae

Malpighian tubules are the insect equivalent of mammalian kidneys and normally drain into the gut at the junction between the mid and hind gut. The Malpighian tubules of the fruit fly Drosophila melanogaster are increasingly being used as a model for studying human renal tract development, histology, nephrolithiasis and urolithiasis. In the present study we report when using X-ray micro-computer tomography techniques, the larval, intrapuparial and adult stages of the larger Calliphora vomitoria can contain large amounts of calcium-rich concretions which are tightly packed in the lumen of both anterior Malpighian tubules. We show that it is feasible to utilise these calcium-rich concretions as a form of marking agent to delineate the various developmental stages of the Malpighian tubules including the crucial phase when the Malpighian tubules reconnect with the hind gut. In the majority of cases during the intrapuparial period the ureters of the Malpighian tubules did not start to re-canalise and thus reconnect with the developing hind gut until the 7^th day of the 10-11 day. Just prior to ecdysis, virtually all the radio-opaque concretions in the Malpighian tubules had emptied into the hind gut and had then been completely excreted by the time the imago emerged from its puparium. In contrast, we show that in flies developing from larvae previously stained by ingesting Rhodamine B, a known substrate for both the Multi Xenobiotic Resistance and Multi Drug Resistant membrane transport systems, the efficiency with which these calcium-rich concretions are excreted by the imago as it emerges from its intrapuparial period can be significantly impaired. Therefore, it might be useful to include C. vomitoria as a model when studying renal tract development and urolithiasis using X-ray micro-computer tomography.

Micro-CT visualization of a promastigote secretory gel (PSG) and parasite plug in the digestive tract of the sand fly Lutzomyia longipalpis infected with Leishmania mexicana

Leishmaniasis is a debilitating disease of the tropics, subtropics and southern Europe caused by Leishmania parasites that are transmitted during blood feeding by phlebotomine sand flies (Diptera: Psychodidae). Using non-invasive micro-computed tomography, we were able to visualize the impact of the laboratory model infection of Lutzomyia longipalpis with Leishmania mexicana and its response to a second blood meal. For the first time we were able to show in 3D the plug of promastigote secretory gel (PSG) and parasites in the distended midgut of whole infected sand flies and measure its volume in relation to that of the midgut. We were also able to measure the degree of opening of the stomodeal valve and demonstrate the extension of the PSG and parasites into the pharynx. Although our pilot study could only examine a few flies, it supports the hypothesis that a second, non-infected, blood meal enhances parasite transmission as we showed that the thoracic PSG-parasite plug in infected flies after a second blood meal was, on average, more than twice the volume of the plug in infected flies that did not have a second blood meal.

Interpreting Morphological Adaptations Associated with Viviparity in the Tsetse Fly Glossina morsitans (Westwood) by Three-Dimensional Analysis

Simple Summary

Tsetse flies, the sole transmitters of African Sleeping Sickness parasites, have a unique reproductive biology. They only develop one offspring at a time, they carry that offspring in their uterus for its entire immature development and provide nourishment for that offspring via milk-like secretions. This specialized reproductive biology has required dramatic modifications to the morphology of the reproductive organs in these and related flies. Here, we use phase contrast micro-Computed Tomography (Micro-CT) to visualize these adaptations in three dimensions for the first time. These adaptations include cuticular modifications allowing increased abdominal volume, expanded abdominal and uterine musculature, reduced egg development capacity, structural features of the male seminal secretions and detailed visualization of the gland responsible for synthesis and secretion of “milk” to feed intrauterine larvae. The ability to examine these tissues within the context of the rest of the organ systems in the fly provides new functional insights into how these changes have facilitated the evolution of the mating and reproductive biology of these flies.

Abstract

Tsetse flies (genus Glossina), the sole vectors of African trypanosomiasis, are distinct from most other insects, due to dramatic morphological and physiological adaptations required to support their unique biology. These adaptations are driven by demands associated with obligate hematophagy and viviparous reproduction. Obligate viviparity entails intrauterine larval development and the provision of maternal nutrients for the developing larvae. The reduced reproductive capacity/rate associated with this biology results in increased inter- and intra-sexual competition. Here, we use phase contrast microcomputed tomography (pcMicroCT) to analyze morphological adaptations associated with viviparous biology. These include (1) modifications facilitating abdominal distention required during blood feeding and pregnancy, (2) abdominal and uterine musculature adaptations for gestation and parturition of developed larvae, (3) reduced ovarian structure and capacity, (4) structural features of the male-derived spermatophore optimizing semen/sperm delivery and inhibition of insemination by competing males and (5) structural features of the milk gland facilitating nutrient incorporation and transfer into the uterus. Three-dimensional analysis of these features provides unprecedented opportunities for examination and discovery of internal morphological features not possible with traditional microscopy techniques and provides new opportunities for comparative morphological analyses over time and between species.