Morphology of water beetles of Russia

Gyrinus sp. (Gyrinidae) Fig. by A.E. Brinev

(fragment of the Key to
freshwater invertebrates of
Russia and adjacent lands)

Body normally with strongly sclerotized integuments, including the elytra covering the membranized tergites of the abdomen. Head capsule (epicranium) is partly drawn into prosternum sclerite, normally without traces of division into separate sclerites, except for the clypeus, frequently divided in front of the forehead, and sometimes also remains of V-shaped epicranial suture. Compound (facetted) eyes are well developed in some cave, underground and parasitic forms); ocelli in the majority of cases are absent; if developed they are situated on the frons (sinciput). On the under surface of the capsule the part adjoining mentum (chin) and not infrequently separated by sutures is termed submentum, and sutures (gular) continuing up to tentorial pits permit to separate throat (gula).

The mouthparts situated in front of the clypeus, genae and gula are of biting type or modified. The labrum, if developed and not fused with the forehead is capable of being drawn in. Situated beneath are strongly sclerotized mandibles bearing denticles on the internal margin, and chewing surface (mola) on the underside, sometimes aquatic forms (usually carnivorous) have grooves or canals for injection of enzymes in extraintestinal digestion. Situated below are maxillae (lower jaws) consisting of the basal plate (cardo), stem (stipes) and, adjoining the latter, 4-segmented palpus and 2 chewing lobes (one of these may be reduced): internal (lacinia) and external (galea). The labium consists of mentum and, situated in front of it, ligula with with two paraglossae and 2 three-segmented palpi.

Segments of palpi are termed palpomeres. Antennae are normally 11-segmented (sometimes 6-10-segmented) and sitting in antennal pits on the forehead on cheeks, quite diverse in shape of segments comprising them, however the first antennomere (scape) is most frequently elongated or somewhat bent.

Antennal segments are termed antennomeres. Thorax includes comparatively separated and moveable prothorax and also immoveable meso- and metathorax or pterothorax, each segment includes modified or additionally divided sclerites: notum, sternite and pleura, although propleura of the majority of species of the suborder Polypahaga and also inintial sternites of mesothoracic and metathoracic imaginal segments of all beetles are placed inside. The sides of the pronotum are partly bent on the ventral side (hypomera) and are separated from protruding propleura by the notopleural suture or in Polyphaga with hidden pleura they are separated by the notosternal suture from prosternum, which is frequently terminated by the intercoxal root participating in the mechanism coordinating movability of prosternal segment in relation to pterothorax.

Mesopleura are normally divided into the anterior sclierite -- mesepisternum and hind sclerite -- mesepimeron, and longitudinal metasterns are situated on each side of metasternite. Mesothorax bear elytra (forewings) and scutellum protruding between them. Sides of elytra are bent on the ventral side in the form of fold termed epipleura. The membranized metanotum with hind membranous wings (sometimes reduced) is covered from above by elytra. The hind wings in repose are folded beneath the elytra.

Legs are articulated with thoracic sclerites in coxal cavities, which may be closed (if hypomeres are fused with of prothorax behind or mesepimeron is separated from the external margin of the mesocoxal cavities, not closed or incompletely closed (metacoxal cavities are always not closed, or open). The legs consist of coxa, dipped in coxal cavity and sometimes bearing appendage (trochantin), trochanter, femur, tibia and 3-5 segmented tarsus, terminating by paired claws. In some aquatic forms initial segments of running legs are flattened and extend transforming into swimming legs.

The abdomen consists of 5-7 visible sclerotized sternites (ventrites) and normally 7-9 tergites (membranized beneath elytra or sclerotized, if these remain uncovered); halteres are situated on the latter, the tergite of the last segment is termed pigidium, and sternite -- hypopygidium. The part of the abdomen drawn inwards forms sclerites of the external genital apparatus. The males aedeagus includes penis trunk (medial plate) with the internal apodema (or two apodemas, but sometimes without them) and membraneous inner sac of penis drawn inside, often bearing characteristic armature of small sclerites, and also parameres and phalobase (basal sclerite), often fusing in tegmen (lateral lobes). The segment preceding the aedeagus may form genital capsule of the anal sclerite above and also ventral plate and spicula gastrale frequently connected. In females the genital organs consist of the ovipositor, dipped in the genital capsule often having ventral spicule with branching posterior end. I consists of valvifer (hemisternites), normally bearing styles at the apex (vaginal palps) and gonocoxites (gonobase).

Oval, rounded or elongated, normally in light coloration with thin cover without distinct sculpture. Eggs are laid in water in tissues of aquatic plants or in humid areas near water by one or in groups. The superfamily Hydrophiloidea is characterized by deposition in silky cocoon, produced by a female.

Body elongated (sometimes oval), frequently narrowed backwards, normally with soft (in hidden living forms), or weakly sclerotized (in open living forms) integuments, but head is always strongly sclerotized with mouthparts of biting type; most frequently weakly convex or somewhat flattened. Colouration is often light, but with a certain degree of darkening of more sclerotized areas, particularly in open living species. Three types of aquatic larvae are distinguished: (1) actively moving predators with large head, long crescent mandibles, developed swimming hairs on the legs and breathing atmospheric air. (2) non-predatory freely moving larvae with shorter appendages, having gills for consuming oxygen dissolved in water; (3) larvae inhabiting tissues of plants with weakly sclerotized body and strongly reduced appendages (eruciform) or devoid of legs (apodous) consuming oxygen from airways of water plants.

Cranium is usually prognathous, less frequently hypo- or even opistognathous in leaf beetles and weevils the prognathous head is retracted inside prothoracic segment; normally retains pronounced epicranial sutures (coronal at the base and frontal at the apex) and also frontoclypeal suture; on the inner side parts of sutures may be continued by a considerable thickening (endocarina). The clypeus may be divided into postclypeus and anteclypeus. 1-6 stemmata are situated on each side in front of 3-4-segmented antennae (sometimes secondarily divided): Carabidae, Dytiscidae, Scirtidae). 2nd segment of antennae (pedicel) on the apex has a thickened sensillum (sensorium), and the last segment (flagellum) usually bears a long seta. The labrum is articulated with clypeus by clypeolabral suture and has characteristic armature on labroepipharyngeal surface. Mandibles frequently have reduced mola, diverse structure of prostheca and adaptations to extraintestinal digestion (particularly well developed in predators). Maxillae consist of small basal sclerite (cardo), very large stipes, sometimes terminate by two lobes (galea and lacinia), only galea, but frequently mala (fused galea and lacinia) and articulated palp (between cardo and stipes there is a distinct articulating area). Mentum is divided into prementum, mentum and postmentum; postmentum (submentum) is frequently fused with gula into gulamentum; ligula is represented by glossae, though without paraglossae. Hypopharynnx often bears specific sclerotized areas (scleroma) with adjoining sclerite (hypopharyngeal bracon) and with paired sclerotized constrictions at the base of the mentum (hypopharyngeal rods).

Legs in Adephaga (and also Archostemata) consist of coxa, trochanter, femur, tibia, tarsus, terminating by paired and less frequently non-paired ungulus, whereas in Myxophaga and Polyphaga the tibia is ended by a sharpened segment that is termed tarsus and sometimes tarsungulus.

Abdomen initially 10-segmented, with the first 7-8 segments approximately similar, the 9th and frequently the 8th are differently modified, the 10th is usually strongly reduced and shifted to the ventral side as an ambulatory device (frequently termed hypopod), but in Staphylinidae it is elongate, and in some Haliplidae it is drawn into the medial process. The 9th tergite frequently bears unsegmented pregomphi (preapical urogomphi) and unsegmented or segmented (apical) urogomphi.

Spiracles, if completely developed are situated on mesothoracic and 1-8 abdominal segments. The initial peripneustic respiratory system is often transformed in aquatic larvae or some their ages into hemipneustic with a reduced number of abdominal spiracles (Hygrobiidae, Haliplidae, etc.); or amphipneustic respiratory system is formed with spiracles on the metathorax and on the abdominal segment 8 (some Psephenidae), metapneustic system with the only pair of functioning spiracles [Noteridae, Amphizoidae, Dytiscidae, some Psephenidae, Scirtidae, Ptilodactylidae, some Chrysomelidae (Donaciinae)] or apneustic system without functioning spiracles (Gyrinidae, Hygrobiidae, some Haliplidae, Hydrophilidae (Berosini), Elmidae, some Psephenidae). However Noteridae in the last larval instar and Microsporidae and Torridincolidae at all stages have 1-8 pairs of abdominal spiracles, but without thoracic spiracles, and in the last stage of Gyrinidae halteres are situated only on 1-3 abdominal segments. In many aquatic forms spiracles are replaced and supplemented by gills of different origin and various structure. In larvae of the 3d instar mesothoracic and abdominal spiracles begin functioning. These permit larvae to leave water and begin functioning. On the whole tracheal gills are distinguished including trachea or tracheoles and gills filled with blood vessels; spiracular gills connected with spiracles; anal gills with osmoregulation functioning.

Pupae are normally soft, light and free; compressed dorso-venrtally, not bent or slightly bent, nearly always wrinkled. Surface is normally with long processes or knobs, bearing setae, and also not infrequently with very large setae that serve for supporting the pupa in the needed position in relation to the walls of pupal cell or cocoon. The segmentation of the body into sclerites more or less intermediate between larval and adult segmentations although the boundaries between the sclerites are not quite distinct. The head is bent downwards, so that above it is covered by the anterior edge of the pronotum, with antennae directed sideways including 9 separated antennomeres and also with palpi directed backwards. Legs are folded with femora and tibiae drawn close to one another, and tarsi oriented backwards. Hind wings and elytra are underdeveloped and are shifted considerably towards the ventral side. The abdomen normally includes 9 tergites and 8 sternites and its apex often with paired processes reminiscent of larval urogomphi. The number of spiracles is reduced in comparison with larvae and adult beetles.