The adult head morphology of the hessian flyMayetiola destructor(diptera, cecidomyiidae)

Abstract

The adult head of the Hessian fly Mayetiola destructor was examined and described in detail. Morphological features are evaluated with respect to phylogenetic implications and possible effects of miniaturisation. Preserved groundplan features of Diptera are the orthognathous orientation of the head, the vestiture of small microtrichia (possible autapomorphy), filiform antennae inserted frontally between the compound eyes, the presence of a clypeolabral muscle (possible autapomorphy), the presence of labellae (autapomorphy), and the presence of only one premental retractor. Potential synapomorphies of the groups assigned to Bibionomorpha are the origin of M. tentorioscapalis medialis on the frons and the loss of M. craniolacinialis. Further apomorphies of Cecidomyiidae identified in Mayetiola are the unusually massive anterior tentorial arm, the absence of the labro-epipharyngeal food channel, the absence of the lacinia, and the presence of antennal sensilla connected by a seta, a feature not known from any other group of Diptera. The very large size of the compound eyes (in relation to the entire head surface) and the complete loss of ocelli are possible effects of miniaturization. The large size of the brain (in relation to the cephalic lumen), the unusual shape of the optic lobes, and the absence of the frontal ganglion as a separate structure are probably also linked with size reduction.

The evolution of head structures in lower Diptera

“…Peterson [41] postulated the presence of a postgenal bridge in the groundplan of Brachycera, but not as an ancestral feature of the entire order. Its presence in several families of lower Diptera (e.g., Deuterophlebiidae, Nymphomyiidae, Tipulidae, Axymyiidae, Bibionidae, and Sciaridae; [28,30,50,58,70]) rather suggests that it may be a dipteran groundplan feature. Hennig [8] discussed a possible correlation between the presence of a hypostomal bridge and the reduction of the tentorium.…”

“…However, the number of antennomeres varies strongly among the groups of lower Diptera, from 5 in Nymphomyiidae [50] to 113 in some Psychodidae [8], and it is strongly reduced in the groundplan of Brachycera [59,60,97,98]. Aside from the number of segments, the antenna is filiform and inserted frontally between the compound eyes in the dipteran groundplan [e.g., 30,31,41,50,58,69] ( Figure 8A, B). Closely adjacent antennal insertions on the frontal region are also found in Mecoptera (with the exception of Caurinus; [80][81][82]115]).…”

“…The club-shaped condition is due to the enlargement of the first flagellomere in Nymphomyiidae and in the brachyceran Drosophilidae, Tabanidae, and Bombyliidae (where the antenna is filiform, but also shortened; [60] Adults of several groups of lower Diptera are short-lived (e.g., Deuterophlebiidae, Nymphomyiidae, and Cecidomyiidae) and have to find their mating partner in a short time span. Therefore, the antenna is well developed and bears numerous olfactory sensilla (see [30]). Interestingly, the antennae of adult Brachycera are greatly shortened, compared to the presumptive dipteran groundplan, but apparently still equipped with a highly efficient set of sensilla (e.g., Drosophila; [116]).…”

“…However, this interpretation appears questionable as M. craniolacinialis (M. 19) is preserved in some dipteran lineages (e.g., Culicidae, Simuliidae, and Bombyliidae; [59,60,95]) as in Mecoptera (Nannochorista, Caurinus, Bittacus, and Merope; [80,82]), whereas a muscle of the galea is consistently absent. The complete reduction of the maxillary endite lobes is an apomorphic feature occurring in Tipulidae, Limoniidae, Cecidomyiidae, some Mycetophilidae, Axymyiidae, and Anthomyiidae (see [30,31,70]). It is apparent that this condition evolved several times independently.…”

“…The reconstruction of the phylogeny and character evolution in Diptera was strongly impeded by the lack of well-documented morphological data. A series of studies on larval and adults head structures were published in the framework of a recent project on dipteran morphology and evolution [e.g., [27][28][29][30][31][32][33], mainly based on innovative techniques optimized by the Entomology Group in the Phyletisches Museum (Institut für Spezielle Zoologie und Evolutionsbiologie, FSU Jena) in the last years [e.g., 34]. An extensive data set was compiled for adults and immature stages.…”