Rollin' rollin rollin' - the diverse inhabitants of a willow leaf

Over the last year or so (yup,  the 'Ecology Spot' is nearing its first birthday!) I've posting several musings on the inhabitants of galls - not only the gall causers but also other species that can make galls their home. However, there is something conceptually similar, and closely related biologically, that I haven't looked at yet, and that is leaf-rollers.

Leaf-rollers, as the term suggests, are organisms that roll leaves to form shelters - unlike galls, the plant doesn't grow new structures, although there is some overlap (excuse the pun) with some leaf-rolls including further distortions such as thickening and increases in cell number, and hence being considered galls (Redfern & Shirley, 2011). In this case, the roll in a willow (Salix) leaf (found at Winnall Moors on the edge of Winchester, southern England) involves twisting and thickening of the plant tissues and so is classed as a gall (only just - sometimes the thickening is very slight).

Willow leaf roll showing twisting and thickening (increased cell size is visible in places)

 The downwards roll affects both sides of the leaf and most of the blade which indicates that it was caused by one of the Phyllocolpa species, a genus of sawflies in the family Tenthredinidae. Indeed, there were small dark sawflies with yellowish legs in the area which may well have been Phyllocolpa, although I did not manage to catch a specimen (either physically or photographically). As the larvae of these sawflies drop to the ground to pupate in the soil, it is not uncommon for galls to be empty, but the only way to find out is to look...

The roll unrolled - at first glance there is nothing but frass (invertebrate faeces)

Looking more closely, a few pale creamy-white rounded-oval eggs can be seen, each no more than 0.5mm in diameter. Whatever they are, an adult invertebrate was here recently. We'll return to these later...

A small cocoon of tangled silk. The lid (top right) is detached showing that whatever developed here has now left.

The bottom of the cocoon showing attachment threads plus a dark patch that may be frass.

So, we have eggs and an empty cocoon - is there anything more immediately identifiable? Well, fortunately yes. Whether or not they are in any way related to the cocoon I don't know, but two exuviae ('skins') were also present.

An unidentified exuvium - probably one of the Hemiptera (true bugs) with clearly defined wing buds and abdominal segments.

That's more like it - something recognisable, an exuvium of a small spider; even the leg hairs are clearly visible.

Although the 'bug' had long gone, the spider (or at least a spider) remained.

Dorsal view of the tiny spider (a few mm long) found in the leaf roll.

A close-up showing various appendages and some of the eyes.

Although the abdominal pattern seems quite clear, this is probably a juvenile. From the general form, I suspect it is one of the orb-weavers (Araneidae) though I would be more than happy if an arachnologist could clarify! If it is this family, it may be using the leaf roll for shelter as hunting takes place on an orb-web (a small one may have been present but un-noticed of course). Along with the spider I also found the following insect already dead (the abdomen was dry and wrinkled) - I wonder if it had become spider-food?

Ventral view of the insect showing the pointed 'snout', long antennae and spotted wings held in a tent-like position.

Dorsal view of the head showing protuberant compound eyes and orange simple eyes (ocelli). Note the patterning on the head and the bristly antennae.

This is a psocid or barklouse - the tented wings with spots at the tips of the wing veins, plus the habitat (on  foliage of trees and shrubs) make this a straightforward identification as the genus Ectopsocus. It is probably the common E. briggsi, but dissection is needed to separate it from E. petersi and E. meridionalis with ceryinaty, and the taxonomy of these closely related species remains uncertain (New, 2005). The protuberant, almost stalked, eyes mean it is probably a male.

And so, that brings us to the end of the specimens that could be identified (to some extent at least) at the time of collection about two weeks ago. However, I did mention that we would return to the eggs. Once found, I put them on their section of leaf in a small container to see if they would hatch, and they did...

Tiny larva about 1mm long - dorsal view

Ventral view showing segmented legs.

Ventral close-up of the head and thorax.

The dorsal view looks superficially somewhat like a woodlouse (i.e. it is 'onisciform'), but the darkened and hardened ('sclerotised') head with small pointed mandibles, plus the legs, show that it is actually a tiny beetle larva, one of two that hatched. It may not be immediately obvious, but this is actually a larva of a 'flea beetle', one of the small species of jumping beetles, the tribe Alticini within the family Chrysomelidae, although it might be within the wider subfamily Galerucinae within which the Alticini are included. The legs are short and have 4 segments (with a claw) and the mandibles are simple and sickle-shaped without a grinding surface ('mola'). There are thin bristles around the body (e.g. just visible top left in the 3rd photo) but no long cerci (tail-like appendages), the antennae are tiny and conical, and willows are a favourite host plant (Cooter & Barclay, 2006). Having just hatched, this is a first-stage larva and hence difficult to identify further, though van Emden (1942) is a standard work that is often useful for larval identification to family. However, the Macro-invertebrate Lab at the City Valley State University have kindly produced a Digital Key to the Aquatic Insects of North Dakota. They have good clear images of a more fully developed larva of this type, and also images of larva at genus level for Pyrrhalta - very similar to what was found here.

So, at present, I have identified all the organisms within this single leaf roll as far as I can. I hope it shows the importance of such small-scale habitats and the diversity they support, as well as highlighting some groups that are likely to be under-recorded due to their small size and tendency to hide. These under-recorded groups and habitats are worth taking the time to investigate as there are discoveries to be made that might be in your local patch of habitat or even your garden or back yard. Happy bug-hunting!


References

Cooter, J. & Barclay, M.V.L. (eds.) (2006). A Coleopterist's Handbook (4th ed.). AES, Orpington.
New, T.R. (2005). Psocids. Psocoptera (Booklice and Barklice) (2nd ed.). RES Handbooks for the Identification of British Insects 1(7): 1-145.
Redfern, M. & Shirley, P. (2011). British Plant Galls (2nd ed.). FSC, Shrewsbury.
van Emden, F.I. (1942). Larvae of British Beetles. III. Keys to the families. Entomologists' Monthly Magazine 78: 206-272.