The tracheal system in post‐embryonic development of holometabolous insects: a case study using the mealworm beetle

Abstract

The tracheal (respiratory) system is regarded as one of the key elements which enabled insects to conquer terrestrial habitats and, as a result, achieve extreme species diversity. Despite this fact, anatomical data concerning this biological system is relatively scarce, especially in an ontogenetic context. The purpose of this study is to provide novel and reliable information on the post-embryonic development of the tracheal system of holometabolous insects using micro-computed tomography methods. Data concerning the structure of the respiratory system acquired from different developmental stages (larvae, pupae and adults) of a single insect species (Tenebrio molitor) are co-analysed in detail. Anatomy of the tracheal system is presented. Sample sizes used (29 individuals) enabled statistical analysis of the results obtained. The following aspects have been investigated (among others): the spiracle arrangement, the number of tracheal ramifications originating from particular spiracles, the diameter of longitudinal trunks, tracheal system volumes, tracheae diameter distribution and fractal dimension analysis. Based on the data acquired, the modularity of the tracheal system is postulated. Using anatomical and functional factors, the following respiratory module types have been distinguished: cephalo-prothoracic, metathoracic and abdominal. These modules can be unambiguously identified in all of the studied developmental stages. A cephalo-prothoracic module aerates organs located in the head capsule, prothorax and additionally prolegs. It is characterised by relatively thick longitudinal trunks and originates in the first thoracic spiracle pair. Thoracic modules support the flight muscles, wings, elytra, meso- and metalegs. The unique feature of this module is the presence of additional longitudinal connections between the neighbouring spiracles. These modules are concentrated around the second prothoracic and the first abdominal spiracle pairs. An abdominal module is characterised by relatively thin ventral longitudinal trunks. Its main role is to support systems located in the abdomen; however, its long visceral tracheae aerate organs situated medially from the flight muscles. Analysis of changes of the tracheal system volume enabled the calculation of growth scaling among body tissues and the volume of the tracheal system. The data presented show that the development of the body volume and tracheal system is not linear in holometabola due to the occurrence of the pupal stage causing a decrease in body volume in the imago and at the same time influencing high growth rates of the tracheal system during metamorphosis, exceeding that ones observed for hemimetabola.

Revealing the respiratory system of the coffee berry borer (Hypothenemus hampei; Coleoptera: Curculionidae: Scolytinae) using micro-computed tomography

“…In recent micro-CT studies of the mealworm beetle tracheal system 9,36 the minimum diameter of tracheal tubes that could be reconstructed was 25 µm, which is significantly greater than the smallest lumina we reconstructed (1.99 µm), and also greater that the largest lumina we reconstructed (21.92 µm) in the coffee berry borer, which is ten times smaller. However, the largest tracheal tubes of the mealworm beetle have approximately 20 times larger diameters (400 µm) than those of the coffee berry borer, the largest of which are the dorsal and ventral prothoracic trunks that are collapsible to facilitate ventilation (Fig.…”

“…Avoiding large diameter tubes could be an adaptation in such a small insect as the coffee berry borer. In fact, to our knowledge, no collapsible tracheae have been reported in mealworm beetles in any studies of its tracheal system 9,31 .…”

“…To assist in ventilation, many insects have air sacs (dilations of the tracheal tubes) that force ventilation when they are compressed by movement of the surrounding muscles 1,4,6,8,10,12–17,24,25,44 . However, some coleopteran species have reduced or no air-sacs at all 9,29,31 . Our study shows that the coffee berry borer has very small air sacs in the abdominal lateral trunks, close to the spiracles; these were particularly visible close to the third and fifth abdominal spiracles, both with light microscopy (Fig.…”

“…X-ray techniques were valuable in demonstrating forced respiration in beetles by compressive movements of the tracheal tubes 3,7,31 , and also in quantifying aspects of tracheal hypermetry in grasshoppers 32 . Most recently, micro-computed tomography (micro-CT) has become the most important non-destructive technique useful in revealing important anatomical insights on the complex anatomy of tracheal tubular systems in Diptera 10,33 , Orthoptera 34,35 , Coleoptera 9,36,37 and non-insect arthropods such as camel spiders (Solifugae) 38 .…”

Three-dimensional reconstruction of a whole insect reveals its phloem sap-sucking mechanism at nano-resolution

“…The volume of total tracheae in N. lugens accounted for about 0.12% of the body volume. Compared to the tracheal density of 0.5% in Tribolium castaneum (Coleoptera: Tenebrionidea), 4.8% in Eleodes obscura (Coleoptera: Tenebrionidea) calculated using the stereological point count method ( Kaiser et al, 2007 ), and 0.6% in Tenebrio molitor (Coleoptera: Tenebrionidea) larva ( Raś et al, 2018 ), the tracheal density in N. lugens is relatively low. This is in agreement with the hypothesis that larger insects invest more on the tracheal system ( Callier and Nijhout, 2011 ; Kaiser et al, 2007 ).…”