Frolov, A.V., Akhmetova, L.A. 2006. Description of the Third-instar Larva of Aphodius bimaculatus (Laxmann) (Coleoptera, Scarabaeidae). Entomological Review, 86, 433-437.
Original Russian text: Фролов, А. В. Ахметова, Л. А. 2006. Описание личинки 3-го возраста Aphodius bimaculatus (Laxmann) (Coleoptera, Scarabaeidae). Энтомологическое обозрение, 85(1): 170–175.
Description of the 3-instar larva of Aphodius bimaculatus (Laxmann) (Coleoptera, Scarabaeidae)
Zoological Institute, Russian Academy of Sciences, Universitetskaya nab. 1, St.Petersburg 199034, Russia.
Aphodius (Acrossus) bimaculatus (Laxmann, 1770) – one of the largest species of the genus Aphodius Ill. of Russian fauna. It can be easily distinguished from the other species of the genus by its distinctive coloration: black pronotum with orange sides, red-orange elytra with dark apices and black spots, and bicolor, black and orange legs and abdomen.
Georgaphic range of the species includes western part of Russia from Sankt-Petersburg, Yaroslavl, Tyumen, Novosibirsk and Krasnoyarsk in the north to Crimea, Stavropol and Baskunchak Lake in the south (Kabakov, Frolov, 1996). The species occurs up to Tomsk Prov., Krasnoyarsk Terr. and Altai Mnt. in the east. It was recorded from most of Europe (except for Southern and South-Western), Ukraine, Northern Kazakhstan and Kirgizia. Recently it was found in Eastern Kazakhstan (Zinchenko et al., 2002).
The beetles are rare throughout the range of the species. Most of the museum specimens, especially those from Western Europe, were collected more than 50 years ago. The range of the species decreases and now it is very rare or became extinct in localities were it was relatively common in the first half of the 20th century.
At present, A. bimaculatus is included in the Red List of Russian Federation in the category 2 – endangered species (Nikitskyi, 2001). The reasons for reduction of its populations are unknown but it is possible that they are not related to threatening of the habitats because this species mostly inhabit pastures and feed on cattle and horse dung. More likely, in our opinion, that this species is more sensitive to toxic chemicals used in agriculture. This is indirectly supported by the fact that the reduction of populations, at least in Russia, corresponds to the beginning of the wide utilisation of chemicals in agriculture. Negative impact of these chemicals, particularly pesticides, on dung beetles is reported in the literature. On-going research in some countries are aimed at reduction of this impact and elimination of the most toxic chemicals from the practice.
A. bimaculatus is known since XVIII century, however its larva was not described due probably to the rarity of the species. In 1995, a population of the species was found in Belarus (Frolov, 1995). The beetles collected in following years were bred in the laboratory and we could examine morphology of the larvae and compare them to the larvae of other Acrossus species.
Beetles and larvae of A. bimaculatus were collected near Rudensk (Belarus, Minsk Distr.) in horse dung. Sixteen 3-instar larvae were examined including 6 reared from the eggs laid by the beetles collected on 29 IV 2000. Remaining larvae were collected in the field on 8 VI 2001. Larvae treatment and preparation follows Frolov (2000).
The work was supported by a grant from Russian Foundation for basic research (no 04-04-49109-а).
Description of 3-instar larva
Head width 3.10±0.14 mm, length (without clypeus and labrum) - 2.24±0.11 mm. Head surface shiny, shagreened, yellowish-brown with unclear pattern of small brown spots and 4 symmetrical dark spots in the middle of frons and on pleural sclerites near frontal sutures. Medial part of pleural sclerites, base and apex of frons darker than remaining part of the head. Frontal sutures are visible as fine lines (рис. 1).
Epicranial suture approximately 2 times shorter than frons height. Each pleural sclerite with 8 long setae: 3 near palpifer, 4 in the centre and 1 near epicranial suture. On the periphery of pleural sclerite there are 3 shorter setae and a few small setae without definite location and number. Head with 5 pairs of setae: 2 short in the centre of frons, 1 short medially and 2 long laterally.
Clypeus trapezoidal, brown, with a pair of long and a pair of very short setae laterally, and a pair of short setae medially. Basal part of clypeus (preclypeus) convex, with symmetrical tubercles, strongly sclerotized (fig. 1, 4).
Labrum (fig. 2) three-lobed, with 38 setae: 16 relatively short setae on the anterior margin, 2 long on dorsal side and 6 short on ventral side of lateral margins, 2 long setae on dorsal surface in the middle. Ventral side of labrum with 4 short setae near anterior margin and 2 basally (fig. 3).
Mandibulae triangular, asymmetrical. Left mandible slightly longer than right one, its scissorial part wider (fig. 5, 6). Base of mandiblae light brown, scissorial and molar part almost black.
Maxillae symmetrical (fig. 7, 8). Cardo with 4 short setae: 2 on ventral side and 2 on lateral margin near base of stipes. Ventral side of stipes with long proximal and short distal setae, dorsal side with a row of 11 stridulatory teeth and 3 short setae near base of palpifer. Palpifer without stridulatory teeth, with one short setae ventrally. Maxillar palpus 4-segmented. Its 1st and 4th segments with 1 seta each, 3rd - with 2 setae. Ventral side of galea with longitudinal row of 12 short setae. Dorsal side and apex of galea with 11 relatively long setae; 4 apical setae slightly curved apically. Dorsal side of lacinia with 7 long and thick setae medially and 3 short setae basally, ventral side with 1 long and thick seta apically and 1 short seta basally. Apex of lacinia 3-dentate.
Legs are about the same size, anterior legs are just slightly shorter than the others. Each leg with 38 seta: coxa with 9 small, trochanter with 1 long and 6 shorter setae, femur with 8 setae, tibia with 13 and tarsus with 2 (fig. 11, 12).
Central part of anal sternite (fig. 13) with 2 rows of relatively long, strongly sclerotized spinules, rounded apically (fig. 16). Rows are about parallel apically and became closer to each other basally. Anal sternite also bears smaller spinules 18 – 25 on each side of medial rows. Lower anal lobe sinuate in the middle and laterally (fig. 14).
In addition to A. bimaculatus, larvae of 5 species of the subgenus Acrossus are described: A. rufipes (L.), A. depressus Kug., A. luridus (F.), (Madle, 1935; Medvedev, 1952; Krell, 1997), A. gagatinus Mén. (Medvedev, 1964) and A. planicollis Reitt. (Dzhambazishvili, Medvedev, 1974). Larvae of all species, including A. bimaculatus, share 2 characters, diagnostic for the subgenus Acrossus: two distinct rows of spinules on anal sternite which differ from the lateral spinules, and modified setae on the abdominal tergites I-V (supporting setae), situated on conical tubercles.
Most of the available descriptions of the larvae are incomplete, however on the base of these data and having examined the larvae of A. depressus and A. rufipes, we can infer that the larva of A. bimaculatus can easily be distinguished from the larvae of other species in having convex clypeus with complex tuberculate sculpture. In lateral view (fig. 4), its clypeus is strongly protruding past general head outline, whereas in other species it is not or very feebly protruding. Larva of A. bimaculatus is similar to that of A. rufipes in having light brown colour of head, but differs from it in having 4 small dark spots in the middle of the frons and on the pleural sclerites near frontal sutures as well as in having almost indistinct reddish tint. Heads of larvae of other species are darker, brown to black-brown.
Median spinules of anal sternite of A. bimaculatus are simple, with rounded or acute apices, similar to those of the larvae of A. depressus, A. luridus and A. planicollis (fig. 15, 16) but differs from those of A. rufipes and A. gagatinus, which have part of spinules wider and laciniate apically (fig. 17, 18).
Other morphological characters used in the diagnostic keys to the larvae of the subgenus Acrossus (Madle, 1935; Medvedev, 1952; Krell, 1997) are less reliable. In particular, the number of medial anal spinules is overlapping in all species and the shape of the rows vary. The shape of frontal fossae vary considerably, unclear and in most cases can not be described in the form suitable for comparison.
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