Lethrini Mulsant & Rey, 1871

POLYPHAGA Emery, 1886

GEOTRUPIDAE Latreille, 1802

Lethrus Scopoli, 1777

Approx. 120 (worldwide)

5-8mm

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Sometimes considered to be a subfamily of the Geotrupidae this tribe includes only a single genus (although other genera are included in a broader subfamily definition), Lethrus Scopoli, 1777, and with more than 120 species it is the largest genus of the family. The distribution is Palaearctic, extending across Europe and into the middle of Asia, many are thermophilic, occurring at lower altitudes, and none have been recorded in the UK.  They are very distinctive; generally large, 15-20mm, with short, transverse elytra which lack striae, and hugely developed head and pronotum. The eyes are convex and protruding and generally divided by a longitudinal canthus which is sometimes widely dilated. Most are drab, black or dark blue but some are metallic green or bronze e.g. L. geminatus Kraatz, 1882. The legs are adapted for digging with transverse ridges to the mid- and hind-tibiae, but they are generally elongate and rather slender, especially the fore-tibiae which are sometimes only weakly toothed along the external edge, and the tarsi are usually long and thin. Sexual dimorphism is well developed; in the male the mandibles have ventral lobes which are sometimes exaggerated and toothed at the apex e.g. L. raymondi Reitter, 1890. These mandibular modifications are used for fighting; males face each other upright with the long front legs extended and the head held back so that they can use the mandibular lobes as weapons. Atypically for geotrupids they are vegetarians; pairs form up in the spring and after mating they cooperate in excavating burrows, typically around 45cm deep with lateral brood chambers into which the female will lay a single egg. They then cooperate in collecting food for the larvae, initially both partners climb nearby stems and begin cutting off leaves which fall and accumulate around the plant, they are then gathered and taken to the burrow, as this continues only the male will gather leaves, passing them into the burrow to the female which will form them into elliptical balls and push them into the brood chambers. The male will clear an area of a metre or so around the burrow and defend it against intruders although during the provisioning work other males will often try to enter the burrow, sometimes succeeding, in which case the resident male will fight them off. When completed the female will seal the burrow with soil and the larvae will develop through the summer and pupate in situ, new generation adults eclose in the autumn and generally remain in the burrow until the following spring. All species are flightless and population peaks occur from March to June, sometimes populations become so large that they may damage commercial crops such as vines, and then insecticides are used to control their numbers. Many populations have declined across Europe in recent decades.

Lethrus rotundicollis 1

Lethrus rotundicollis 1

Lethrus rotundicollis 2

Lethrus rotundicollis 2

Lethrus rotundicollis 3

Lethrus rotundicollis 3

Lethrus rotundicollis 4

Lethrus rotundicollis 4

Lethrus rotundicollis 5

Lethrus rotundicollis 5

Lethrus rotundicollis 6

Lethrus rotundicollis 6