Osservazioni sulla ovideposizione e sul ciclo larvale in Mastigus pilifer Kraatz (Coleoptera, Scydmaenidae)

Abstract

OBSERVATIONS ON OVOPOSITION AND LARVAL CYCLE IN Mastigus pilifer KRAATZ (Coleoptera, Scydmaenidae)

Full-grown larvae of Mastigus pilifer can commonly be found in forest litter without any trace of younger instars, because of their unusual larva! cycle.

The discovery of eggs and the study of internai organs of adults bave led to the following observations on ovoposition, female genital system and behaviour of larvae:

l) Clusters of eggs are laid in autumn under stones and are coated with a white, bright secretion (fig 1-2).

2) This secretion is composed by some hundreds of small spherical capsules, mostly 100-150 !J.m in diameter, arranged in a very long double series; each capsule consists of a thin lined shell containing both an oily and a granulous substance (figs 3-4).

3) The source of such secretion is a pocket-shaped abdominal gland of the females, deriving from the intersegmental membrane between the 7th sternite and the genital segment, and bearing glandular epithelium on the inferior wall (figs 5-11). In females bearing mature eggs the reservoir of this gland is full up with an oily substance.

4) The larvae batch few days after ovoposition and the larva! cycle includes three instars.

5) During the 1st and 2nd instar the larvae do not depart from the secretion mass (fig. 12). The first moult takes piace about 20 hours after hatching, the second nearly 3 days later.

6) The 2nd instar larvae spend part of their time in breaking the secretion capsules, and they probably swallow the secretion.

7) The larvae leave the batch piace only after the second moult; they disperse in forest litter and probably search for alive preys (fig. 13).

The 3rd instar larvae differ from the younger ones in body colour and number of antenna! segments. On head discum they bear a characteristic glandular area, from which they probably get a secretion for cleaning the antennae by means of their fore legs (fig. 14).

It is evident that the larval cycle spares the 1st and 2nd instar larvae the search for food. The hypodermic colouring of the young larvae, which is similar to that of the eggs, suggests that an amount of yolk substances, deposited in the midgut and/or in the fat body, remains available for the larva! development. Furthermore, it is possible that the young larvae get an additional food source from the secretion which coates the egg mass.

Le femmine di Mastigus pilifer posseggono nell'addome una profonda tasca ghiandolare, derivante dalla membrana intersegmentale fra 1'8° urosternite e il segmento genitale, che si riempie di un secreto di aspetto oleoso nel periodo di maturazione delle uova. All'atto della ovideposizione, questo secreto viene emesso sotto forma di una lunghis- sima doppia collana di minuscole capsule sferiche, che si deposita irregolarmente e ricopre l'ovatura.

Le uova sono deposte in autunno, in gruppi, sotto le pietre, e schiudono pochi giorni dopo. Gli stadi larvali sono tre.

La prima e la seconda muta avvengono rispettivamente a circa l giorno e 4 giorni di distanza dalla schiusura. Le larve di I e II età non si allontanano dal luogo di ovideposizione, ma si soffermano sopra la masserella di secreto materno; quelle di II età si dedicano a rompere le capsule di secreto con le mandibole e presumibilmente ingeriscono il liquido oleoso che si spande. Dopo la seconda muta le larve abbandonano i resti dell'ovatura e si disperdono nella lettiera in cerca di prede.

Le larve mature presentano sul capo un caratteristico raggruppamento di corte setole spiniformi, inglobate in un secreto che probabilmente viene adoperato per la pulizia delle antenne.