The duct connecting Malpighian tubules and gut: an ultrastructural and comparative analysis in various Ephemeroptera nymphs (Pterygota)

Abstract

Malpighian tubules proper are connected to the gut by ducts called trunks, the organization of which is described at ultrastructural level in the nymphs of various mayfly species, namely Ecdyonurus venosus (Heptageniidae), Ephemerella ignita (Ephemerellidae), Choroterpes picteti (Leptophlebiidae), and Caenis luctuosa (Caenidae). Trunks are luminal tubes varying in arrangement, number, shape, and length. The main cell type of the trunk wall is represented by cells that are bordered by a thin cuticle along their luminal side (type-A cells). Whereas these cells are exclusive in the long trunks (such as those of C. picteti and C. luctuosa), in the shorter ones (such as those of E. venosus and Ephemerella ignita) cells with a microvillous luminal border (type-B cells) are also encountered. These cells are located close to the entrance of the collecting duct of the Malpighian tubules proper, and their long microvilli form a dense filamentous mesh filling up the lumen. Both cell types exhibit fine structural features that are characteristics of ion-transporting epithelia. Ultrastructural investigations show that trunks are not a simple conducting system but are involved in the regulation of the ionic composition of the primary urine for producing a fluid hypo-osmotic with respect to the hemolymph.

Strategies of ionoregulation in the freshwater nymph of the mayfly (Hexagenia rigida)

“…Interestingly, MTs of insects (including C. riparius and A. aegypti) are well known to secrete a fluid that is iso-osmotic to the hemolymph; however, it was suggested that the trunk of mayfly nymph MTs has a reabsorptive function aiding in production of hypo-osmotic fluid (Gaino and Rebora, 2000a). This idea is based on ultrastructural studies of the mayfly nymph MT trunk which reported that the cells making up this region are morphologically similar to the cells lining the lumen of the HG (Nicholls, 1983;Martoja and Ballan-Dufrançais, 1984;Gaino and Rebora, 2000a). In insects, the primary site of reabsorption of ions, which in FW insects results in a hypo-osmotic fluid to be excreted, is the HG (Cochran, 1985;Phillips et al, 1986;Bradley, 1987).…”

The environmental costs of mountaintop mining valley fill operations for aquatic ecosystems of the Central Appalachians

“…Mayflies in particular are highly sensitive to ionic stress as they regulate their ion uptake and release using specialized structures within their gills, their integument, and internally via Malphigian tubules. 83 For these sensitive taxa, large increases in certain ions can disrupt water balance and ion exchange processes and cause stress or death. Tests for conductivity toxicity for mayflies have often proved inconclusive, 82,84-87 yet these studies are typically based on lab toxicological tests with hardy organisms that are easy to rear in lab settings (i.e., Hexagenia, Centroptilum, Cloeon, Isonychia) and are likely to be less sensitive than the mayfly genera that appear especially susceptible to ionic stress (e.g., ephemerellids, heptageniids).…”

“…This is particularly important for aquatic insects with high cuticular permeability. Mayflies in particular are highly sensitive to ionic stress as they regulate their ion uptake and release using specialized structures within their gills, their integument, and internally via Malphigian tubules 83 . For these sensitive taxa, large increases in certain ions can disrupt water balance and ion exchange processes and cause stress or death.…”

Physiological responses of freshwater insects to salinity: molecular-, cellular- and organ-level studies

“…The Malpighian tubules of mayfly nymphs have also been investigated in the context of osmoregulation, but we do not have a clear understanding of their function. The morphological structure of the Malpighian tubules in mayfly nymphs is comparatively complex (Gaino and Rebora, 2000a). The distal portions of the tubules are coiled to varying extents, and connect to the gut either directly or through connecting tubes and trunks (Gaino and Rebora, 2000a,b;Nicholls, 1983;Nowghani et al, 2017).…”