STUDI SULLE LARVE DEI COLEOTTERI DITISCIDI VI. Studio per fini sistematici del comportamento dei caratteri delle mandibole nelle larve di alcune specie della subf. Colymbetinae

Abstract

SURVEY FOR SYSTEMATIC PURPOSE OF THE MORPHOLOGY OF THE MANDIBULAE IN THE LARVAE OF SOME SPECIES OF SUBF. Colymbetinae (Coleoptera, Dytiscidae)

The author compares the morphology of the mandibulae in some larvae of subf. Colymbetinae, in order to collect data for a systematic ordering of the different larval patterns of the Subfamily.

The behaviour of the characters regarding the mandibular groove is particularly interesting; mandibular groove, in fact, is absent in the larvae of genus Copelatus here examined (fig 2), whereas it is present in the other species with very different features; its lips (dorsal and ventral), in fact, may show no approach each other (fig. 10), otherwise they come near each other in a middle part of their own length without touching togheter (fig. 3-4-5), otherwise they touch each other in a middle part of their own length (fig. 1-6- 7-8-9); some other distinctive features of the mandibular groove regard the behaviour of a pubescence of the ventral lip.

On the basis of the different features, the author distinguishes some patterns of mandibula; different patterns of mandibula may even be found between the first instar larva and the II and III instar larvae, in particular species; on the contrary, the same pattern may even be found in larvae of different genera.

Looking at the differences and similarities among the different patterns as a whole, the author sketches out the hypothesis that those patterns represent some stages of a gradual specialization of the mandibulae of Colymbetinae larvae in the fuction of injecting and sucking liquids; on the basis of this hypothesis the author arranges the different patterns (as shown in fig. 11) in a main evolutive succession (including most of them) and two secondary branches (leading to Ilybius quadriguttatus and Matus bicarinatus patterns).

The main succession begins with the pattern of Copelatus haemorroidalis and glyphicus - which is lacking of mandibular groove - and ends with the pattern of Melanodytes pustulatus - the mandibular groove of which is very specialized; the supposed modalities of the progressive specialization are given; this specialization would moreover carry on its way in the larvae of subf. Dytiscinae. The pattern of Ilybius quadriguttatus would rise - through some other unknown steps - from that of Ilybius fuliginosus. The pattern of Matus bicarinatus is supposed to rise from a now indeterminable pattern included in the main succession.

The differecens that the author finds between the mandibulae of the first instar larva and the mandibulae of the Il and III instar larvae, in particular species, are ascribed to differences in the evolutive pressure; this fact is possible because the quality of the utilizable preys may change in some species (GALEWSKI, 1973 a) according to the larval instar.

Si espongono i risultati di un'indagine preliminare di morfologia comparata sulle mandibole delle larve della subf. Colymbetinae. Tali risultati ven- gono, inoltre, discussi, per cercare una spiegazione alla diversità e alla gradualità che si riscontra nel comportamento dei caratteri di tali organi nelle entità prese in esame. Nei caratteri delle mandibole sembrano risiedere elementi utili per la diagnosi del grado di affinità fra le specie della subf. Colymbetinae. Si avanza una serie di ipotesi, da considerare orientative di future ricerche sull'argomento.