Shining leaf chafers, the subfamily Rutelinae, get their name from the bright, often metallic colors of their adults, which feed on plant leaves. The group is enormous: more than 4,000 species distributed among 235 genera, organized into seven tribes. Yet despite the dazzling adults, the larvae of these beetles, commonly called white grubs, remain largely unknown. Fewer than 2 percent of all known scarab species have had their larvae described, a gap that frustrates ecologists who cannot reliably identify which grub belongs to which beetle, and therefore cannot generalize the ecological role of any given species until its immature stages have been accurately matched.

This is where Parastasia ferrieri comes in. Rutelini, the tribe to which it belongs, encompasses roughly a thousand species in 93 genera, with a primary presence in the New World. Recently, both larvae and adults of P. ferrieri were discovered in Liaoning Province of northeastern China, in the Palearctic realm, similar to previous records in Korea. Parastasia is one of the largest genera within Rutelini, comprising approximately 105 species worldwide, but unlike many of its showy relatives, its adults lack enlarged mandibles, thoracic horns, or vibrant metallic coloration. That modesty may explain why the species has gone largely unnoticed.

The research team collected 13 larvae from Qipanshan Forest Park in Shenyang City in late October 2019. The larvae were reared in plastic boxes filled with moist, fermented sawdust, with a decayed wood log off-cut provided to facilitate boring and egg-laying. Adults emerged the following May, and third instar larvae were collected from the sawdust in September. Ten larvae were fixed in Dietrich’s solution, heated to 70 degrees Celsius and left to cool naturally for 12 hours before preservation in 75 percent ethanol. For scanning electron microscopy, dissected organs underwent serial dehydration in graded ethanol, replacement with tertiary butanol, freeze-drying for 3 hours, and sputter-coating with gold before examination under a Hitachi S-3400N scanning electron microscope at 5 kV.

The larvae are of typically scarabaeiform shape, bearing three pairs of thoracic legs on a C-shaped body. The trunk is generally white, with a yellowish head capsule, thoracic legs, and spiracles. The head capsules measure 3.1 millimeters in width on average and display symmetrical adornment with a total of 14 pairs of setae: two pairs on the clypeus, five pairs in the frontal region, three pairs on the vertex, and four pairs in the genal area. The antenna is elongated and slender, comprising four segments with the second being the longest, and the distal conical segment bears seven sensilla basiconica at its apex.

The mouthparts are of a biting-chewing type, and it is here that the larva reveals its most remarkable features. The labrum, slightly wider than it is long, displays four prominent protuberances on its outer surface, an atypical trait not mentioned in the congeneric larvae of P. brevipes. The epipharynx, the membranous structure on the inner surfaces of the labrum and clypeus, is divided into distinct functional areas including a haptomerum at the apex, a pair of plegmatia on the lateral margins, and glabrous gymnoparia on both sides. Notably, the haptomerum lacks a helus, consistent with previous descriptions in Rutelini, where heli are entirely absent, in contrast to Anomalini with two to four, Anoplognathini with six to eight, and Adoretini with six to nine.

The paired mandibles are heavily sclerotized and symmetric, each bearing two scissorial teeth with apical teeth curved inward. This symmetry is unusual within Rutelini, where larval mandibles are typically asymmetric, equipped with three teeth on the left and two on the right. Each mandible also bears a shuttle-shaped stridulatory area on its ventral surface. The molar region is asymmetric, featuring a prominent molar tooth with an acia on the left and a group of four ridged molar teeth on the right. The maxillae, labia, and hypopharynx are fused into a structural complex, with each maxilla bearing a row of five stridulatory teeth and an anterior truncate process, and the maxillary palp consisting of four segments with ten sensilla basiconica on its apex.

The thoracic legs are composed of five segments each: coxa, trochanter, femur, tibiotarsus, and a distal single claw. The claw is heavily sclerotized with an obtuse distal end, congruent with other saproxylic larvae in Lucanidae, the wood-decomposing beetles. In most Melolonthinae and Cetoniinae, the claw is typically curved, sharp, and adorned with paired setae, while in Rutelinae, thoracic claws are typically sharp in Adoretini, Geniatini, and Anomalini. The blunt claws of P. ferrieri, combined with its other features, point toward a saproxylic lifestyle, feeding on decaying wood rather than living roots.

This correlation between morphology and feeding habits is central to the study’s discussion. Mandibles, being the most heavily sclerotized structures on the mouthparts, are structurally correlated with feeding habits across scarab lineages. Straight and apically sharp incisors appear in wood-consuming species of Lucanidae and Passalidae, sharp but inwardly curved incisors in dung decomposers within Aphodiinae and Scarabaeinae, and blade-like cutting edges in phytophagous Melolonthinae. Within Rutelinae, mandibular incisors are typically blade-like in phytophagous larvae of Anomalini and Adoretini, and apically sharp and curved in saproxylic larvae of Rutelini. The larvae of P. ferrieri, with their symmetric two-toothed mandibles and blunt claws, fit the saproxylic pattern seen in the previously described P. brevipes, which is reported to feed on dead wood.

The significance of this work extends beyond taxonomy. In the Palearctic realm, particularly in northeastern China, larvae of Rutelinae are frequently recognized as agricultural pests because they mostly belong to the phytophagous Anomalini or Adoretini, attacking living roots of crops and trees. The scarcity of records of P. ferrieri may be attributed to their small, nocturnal adults, or the fact that their larvae never attack living organs of crops or trees. Given their biogeographical significance, the researchers argue that the larvae of P. ferrieri warrant increased attention for conservation purposes. By documenting the larval morphology of this Palearctic Rutelini in such detail, the study adds a crucial piece to the puzzle of scarab diversity, helping researchers distinguish saproxylic decomposers from root-feeding pests and ultimately understand the ecological roles these hidden grubs play in forest ecosystems.

Subject of Research: Larval morphology and feeding habits of the Palearctic ruteline beetle Parastasia ferrieri

Article Title: Larval morphology of a Palearctic Rutelini , Parastasia ferrieri ( Coleoptera , Scarabaeidae ), with discussions on their feeding habits

Article References: Sun, X.-Y., Dong, X.-M., & Jiang, L. (2024). Larval morphology of a Palearctic Rutelini, Parastasia ferrieri (Coleoptera, Scarabaeidae), with discussions on their feeding habits. Deutsche Entomologische Zeitschrift, 71(1), 185-192. https://doi.org/10.3897/dez.71.113727

Image Credits: AI Generated

DOI: 10.3897/dez.71.113727

Keywords: Parastasia ferrieri, Rutelini, Rutelinae, Scarabaeidae, larval morphology, scanning electron microscopy, white grubs, saproxylic, feeding habits, Palearctic, China, entomology

The Jialing River stretches 1,345 kilometers and drains roughly 160,000 square kilometers of southwestern China. Its middle reaches, running 633 kilometers from Zhaohua to Hechuan, have been fully channelized by a staircase of thirteen hydropower dams. This cascade construction has slowed the current, homogenized habitats, and transformed prey communities from species adapted to fast-flowing water to those suited to still, reservoir-like conditions. Such profound disturbance raises an obvious question for ecologists: when a river’s plumbing is rebuilt, what happens to the microbial communities that help fish digest their food and fend off disease?

To find out, researchers sampled eight dominant fish species representing three feeding strategies from the channelized midstream between June and July 2024. The carnivorous lineup included the catfish Hemibagrus macropterus and the mandarin fish Siniperca chuatsi. Omnivores comprised Leiocassis crassilabris, crucian carp (Carassius auratus), the spotted barbel Hemibarbus maculatus, and common carp (Cyprinus carpio). Herbivores were represented by Xenocypris davidi and grass carp (Ctenopharyngodon idellus). In total, 38 specimens were collected, dissected under aseptic conditions, and their intestinal contents rinsed, flash-frozen in liquid nitrogen, and stored at minus 80 degrees Celsius until analysis.

The team then sequenced the V3–V4 hypervariable region of the bacterial 16S rRNA gene on an Illumina platform, yielding more than one million high-quality reads. After quality filtering and clustering into operational taxonomic units at 97 percent similarity, the dataset contained 5,784 OTUs, with every sample rarefied to a common depth of 28,274 sequences. Rarefaction curves plateaued at around 15,000 sequences, confirming that the sequencing captured the bulk of microbial diversity. Strikingly, only 98 OTUs—just 1.69 percent of the total—were shared across all eight fish species, underscoring how species-specific these internal ecosystems are. The herbivorous Xenocypris davidi alone harbored 959 unique OTUs, while the predatory mandarin fish had just 22.

Community composition told a clear dietary story. Firmicutes dominated the carnivore guts, exceeding 93 percent relative abundance in both predator species, with the protein-fermenting genus Clostridium_sensu_stricto_1 reaching nearly 70 percent in Hemibagrus macropterus. Omnivorous and herbivorous fish instead showed higher proportions of Proteobacteria, along with notable levels of Actinobacteria. Herbivorous fish were significantly enriched in Cetobacterium, a genus known to synthesize vitamin B12 in freshwater fish intestines—a potentially crucial service, since plant-based diets are typically deficient in this vitamin. Omnivores displayed the most taxonomically balanced communities, with genera such as Romboutsia and unclassified Peptostreptococcaceae consistently abundant across all samples.

Diversity metrics reinforced the pattern. Herbivorous fish posted the highest Shannon-Wiener index (mean 4.42) and Chao1 richness (mean 1,836.7), omnivores fell in between, and carnivores had the lowest values on both counts (Shannon-Wiener 1.21; Chao1 173.99). The differences were statistically significant, with post hoc tests separating carnivores from both other groups. Permutational multivariate analysis of variance showed that feeding habit explained 18 percent of the variation in community structure, while host species identity explained an even larger share at 42 percent. Principal coordinates analysis confirmed that carnivore samples clustered most tightly, suggesting their gut communities are strongly canalized by diet and less sensitive to environmental fluctuations than those of plant-eaters and generalists.

The functional predictions, generated with PICRUSt2 from the 16S data, were where the study’s real surprise emerged. The three expected diet-function associations held firm: carnivorous fish showed the highest relative abundance of cysteine and methionine metabolism, a pathway central to processing high-protein diets; omnivorous fish were enriched in microbial metabolism in diverse environments, reflecting their metabolic flexibility; and herbivorous fish led in carbon fixation pathways in prokaryotes, an adaptation to fiber-rich plant material. Notably, the functional profiles separated the feeding groups even more cleanly than taxonomic composition did, hinting at functional convergence—different microbial communities performing similar ecological jobs.

But two pathways defied expectations. Carnivorous fish guts showed the highest relative abundances of starch and sucrose metabolism (1.83 percent) and the phosphotransferase system, or PTS, a bacterial transport machinery that efficiently scavenges and phosphorylates sugars. This runs counter to the traditional view that predatory fish have limited carbohydrate-processing capacity. The authors propose a plausible ecological explanation rooted in the dams themselves: cascade channelization has shifted benthic invertebrate communities toward lentic species such as chironomid larvae, which are rich in glycogen and other energy-dense carbohydrates. Predators feeding on these altered prey may thus encounter more fermentable sugars than their ancestors did, and their gut microbes appear to have retained—or expanded—the metabolic toolkit to exploit them. The researchers caution that this link between channelization and carbohydrate metabolism remains a hypothesis requiring verification against non-channelized control systems.

The study’s implications extend beyond fish physiology. Because gut microbial communities track both diet and habitat conditions, the authors argue they could serve as bioindicators of aquatic environmental quality and food web health. Monitoring shifts in the gut microbiota of sentinel fish species might offer early warnings of ecosystem degradation and guide targeted conservation strategies. For reservoir management, the findings point toward a concrete recommendation: maintaining hydrological heterogeneity and diverse prey communities within dammed rivers is essential to preserving the healthy expression of gut microbial functions in fish. Restoring the diversity and stability of riverine food webs, the study suggests, is not just about the fish themselves, but about the trillions of microbes that underpin their nutrition, immunity, and adaptation.

The authors are candid about the study’s limitations. The functional profiles are predictions of metabolic potential, not direct measurements of gene expression, and PICRUSt2’s accuracy depends on reference genomes that remain sparse for taxa like Fusobacteria and Cetobacterium. The design lacks a non-channelized comparison river, sample sizes were uneven across feeding groups—grass carp was represented by only two individuals—and the work captures a single season. Future research integrating metatranscriptomics, metabolomics, stable isotope tracing, and seasonal sampling could confirm whether these predicted pathways are actively expressed and disentangle the effects of diet from host evolutionary history. Even so, the study delivers a compelling baseline: in a river remade by concrete and turbines, the ancient partnership between fish and their gut microbes endures, quietly adapting to a menu that humans have rewritten.

Subject of Research: Gut microbiota functional profiles of fish with different feeding habits in a cascade-channelized river

Article Title: Functional Profiles of Fish Gut Microbiota Associated With Feeding Habits in the Midstream of the Jialing River Under Cascade Channelization

Article References: Hu, M., Chen, C., Feng, Y., Liu, L., Tang, X., Wang, H., & Zhang, F. (2026). Functional Profiles of Fish Gut Microbiota Associated With Feeding Habits in the Midstream of the Jialing River Under Cascade Channelization. Ecology and Evolution, 16(10), Article e74414. https://doi.org/10.1002/ece3.74414

Image Credits: AI Generated

DOI: 10.1002/ece3.74414

Keywords: fish gut microbiota, 16S rRNA sequencing, PICRUSt2, feeding habits, cascade dams, Jialing River, Yangtze tributary, channelization, microbial ecology, KEGG pathways, aquatic conservation, Firmicutes