Detailed morphological descriptions of the immature stages of the ant parasite Microdon mutabilis (Diptera: Syrphidae: Microdontinae) and a discussion of its functional morphology, behaviour and host specificity. Corresponding author

The myrmecophilous immature stages of hover fl ies of the genus Microdon Meigen, 1803 (Diptera, Syrphidae) are still poorly known and only about 15 species were previously incompletely described and/or illustrated using light microscopy based on occasional fi ndings mainly of pupae and third instar larvae. The exceptional fi nding of a large number of second and third instar larvae and pupae (159 specimens) of Microdon mutabilis (Linnaeus, 1758) inside the nest of a new host species, Formica cunicularia Latreille,1798, enabled us to rear them and obtain a great number of eggs and fi rst instar larvae. We fi lmed and described the feeding behaviour and locomotion of these highly derived slug-like larvae. Combining light, fl uorescence and scanning electron (SEM) microscopy, we describe in detail and illustrate the external features of all the immature stages of M. mutabilis (eggs, larvae and pupae). Covering the entire chorion of the egg is a peculiar microsculpture composed of volcano-like processes. The three larval instars strongly differ from each other, especially at the level of the shape of the body, the posterior spiracular tubercle and the cephaloskeleton. SEM microscopy was used to describe in detail the microsculpture, sensorial structures, spiracles and cephalic appendages of larvae and pupae. Fluorescence microscopy was used to reveal the exceptional presence of resilin in the external layer of the posterior spiracular tubercle in fi rst instar larvae. The possible functional signifi cance of these structures is discussed.