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Genus Thelaxes

Oak thelaxids

Species Overview: Thelaxes Thelaxes dryophila Thelaxes suberi

Genus Thelaxes [Pterocommini]

The wingless viviparae are small (1 - 2.3 mm) oval and brown or greenish, sometimes with a paler stripe along the back. The rostrum is distinctive in having a long and almost needle-like last rostral segment. The cauda is knobbed and the siphunculi are very short and virtually pore-like.

There are 4 species which feed on the young shoots, leaves and young acorns of various Oak species (Quercus: Fagaceae). Three are in Europe and the Mediterranean region, and one in North America. They have a sexual stage in the life cycle, but do not host alternate. They are usually attended by ants.

 

Thelaxes dryophila (Common oak thelaxid)

The wingless viviparae are oval, rather flattened,dark brownish-red to purplish grey with a paler spinal stripe. The antennae, legs, siphunculi amd cauda are brownish. The antennae are 5-segmented and are slightly less than half as long as the body. The terminal process is less than half the length of the base of the last antennal segment. Hairs on abdominal tergite 5 are spine-like. This last character distinguishes the species from the very similar Thelaxes suberi which has those hairs very thick and dagger-like. Thelaxes suberi mainly occurs on oak species in southern Europe and in UK, but usually not on English oak (Quercus robur).

Winged females have a black head and thorax with the antennae, legs, cauda and areas around the siphunculi dark. The abdomen has dorsal cross bands on the rear segments and dark marginal plates. Unlike most aphids the wings are folded horizontally rather than tent like over the abdomen.

 

The common oak thelaxid does not host alternate and is found on many species of oak (Quercus spp.). Colonies at the tips of the shoots spread on to stems, leaf petioles and along mid ribs on the undersides of the leaves. They are also found on the developing acorns. It occurs in Europe, the Mediterranean region and south-west Asia.

 

Thelaxes suberi (Southern oak thelaxid)

Adult apterae of Thelaxes suberi vary in colour from pale green to dark brown (see pictures below). The terminal process of the sixth antennal segment is 0.26-0.40 times as long as the base of that segment. The body length of adult aptera is 0.9-1.5 mm.

 

Thelaxes suberi siphunculi are very short and the knob of the cauda is at least as long as broad. The hairs on abdominal tergite 5 are mostly more than 25um long, very thick and dagger-like

Thelaxes suberi live on the young shoots, leaves and developing acorns of Quercus spp. especially Quercus cerris and Quercus ilex (but not Quercus robur). Immature sexual forms apparently appear at the beginning of the summer, aestivate through the summer, and then adult oviparae and males occur in autumn. Thelaxes suberi is present in England, southern Europe, Mediterranean region, southwest Asia and South Africa.

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Acknowledgements

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994)  and Blackman & Eastop (2006)  supplemented with Blackman (1974) , Stroyan (1977) , Stroyan (1984) , Blackman & Eastop (1984) , Heie (1980-1995) , Dixon & Thieme (2007)  and Blackman (2010) . We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure  provided by Blackman & Eastop (2006).

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