Described are all the sensilla and non-innervated hairs observable by scanning electron microscopy on the thorax and wings of the minute (0 .4- 0.8 mm) parasitoid wasp,
Trichogramma minutum
. Most of the sensilla appear mechanosensory, and occur on the wing veins, around the wing bases and near the spiracles. The number, position and morphological type of the sensilla were constant between individuals. Dorsally, the thorax bears 18 hair sensilla, between 5 and 50 μm long. Around each forewing base are nine hair sensilla, and three unique sensilla less than 1 μm long; two of the latter form bifurcated ‘pronged’ structures. Of the nine, four form the anterior wing base hairplate, and two the basalare hairplate; these probably contact the wing at extreme anterodorsal and anteroventral positions. Each hindwing base is equipped with three hairplates, of either two or three 4-6 μm hairs. Two of these are located appropriately to contact the wing during flight, the third only when the wings are folded. Posterior to the hindwing base are two socketed hairs and an unusual grooved hair 10—12 μm long, all located near the opening of the third thoracic spiracle. Each membranous forewing is surrounded by 110-120 evenly spaced, unsocketed fringe hairs. A parallel secondary row of similar hairs occurs on both dorsal and ventral surfaces. The forewing surfaces also bear numerous hairs, unsocketed marginally, but with increasingly distinct sockets centrally. Dorsally, these hairs mainly occur in rows radiating from the wing vein. Ventrally the surface hairs are only partly arranged in rows, and vary more in form. Transmission electron microscopy shows these hairs to be non-innervated. In contrast, the wing veins bear innervated structures, comprising 19 hair-like, and 22-23 campaniform, sensilla. The rod-like hindwings also bear a fringe of hairs similar to those of the forewing, although longer and confined to the posterior margin. The sensilla of the hind wing comprise four ventral and one dorsal campaniform sensilla, and six hairs. The function of the described structures is discussed in relation to the flight mechanism and its control in these very small insects.
New fossil wasp species from the earliest Eocene Fur Formation has its closest relatives in late Eocene ambers (Hymenoptera, Ichneumonidae, Pherhombinae)
