Teaching the peanut pirate

A fairly quick observation today... as a wildlife-friendly gardener, I put out bird-food, including peanuts for blue tits and other species. However, birds don't read the labels explaining which food is allocated to which group of species and hence piracy is rife - not that I might - they all have to eat... Anyhow, this morning, I couldn't help notice even more jackdaw (Corvus monedula) activity than usual - given the large jackdaw roosts near here, there's almost always some - in particular a group on our garden fence. Among the usual bickering and squabbling, there was a calmer group of three which appeared to be a pair of adults (black, glossy) with a fledged youngster (some pale feather edges, scruffy neck, less assured behaviour).

A probable family group of jackdaws using a peanut feeder

The adults were feeding - and ejecting other birds that tried to do so - while the youngster watched (or in the above photo got distracted by something off to the right...). In any case, the lesson seemed to work because a couple of minutes later, the adults were perched nearby and the youngster was feeding.

Young jackdaw feeding on peanuts

A simple behavioural observation, but interesting to see in action - so, before I get carried away by the urge to anthropomorphise, I'll stop there - there's much written about the adaptability and learning abilities of corvids, including in one of my early posts here. Enjoy!

A beady-eyed jackdaw in our garden

Tarantula anatomy III: the appendages

Having looked at the abdomen and cephalothorax, it's time for the third and final instalment of tarantula anatomy based on the features visible on a moulted skin.

Ventral view of the sternum showing articulations with legs and chelicerae

Although legs and chelicerae are the most prominent appendages and found on the cephalothorax, I want to start with the spinnerets found at the end of the abdomen. Even though this is not a web-weaving spider, it still needs to produce silk e.g. to form eggs cocoons (this is a female), and does so from the paired spinnerets. Each spinneret is linked to silk glands and is a segmented structure to allow the silk to be manipulated as it is extruded.

One of the spinnerets - note the segmentation and longitudinal groove in the last segment.

Moving forward, a key feature is of course the presence of eight legs - the source of much arachnophobia. These are clearly jointed and are dark with orange-red bands giving this species its common name. However, the part I want to look at is the end of the final segment or tarsus - the spider's 'foot' if you like. Whereas the rest of the leg (and much of the spider) is covered in a variety of bristles, the foot-pad is covered in tiny, short hairs giving it a soft, velvety appearance. More importantly, this also means that the pad has a large surface area, aiding grip - much like the almost fractal convoluted ridges on a gecko's foot (maybe more on that another time - there are electrostatic effects involved which are fascinating), as well as a pair of small claws.

The underside of the tarsus showing a velvety covering of small hairs.

Tip of the tarsus with the tiny claw (one of a pair) indicated.

Moving further forward, the next appendages are of course the mouthparts, in particular the fang-bearing structures called chelicerae.

Ventral view of the chelicerae showing fangs.

The chelicerae are articulated and can flex the fangs forward to grip prey as the fangs are hardened and sharply pointed. As the spider is not strongly venomous, it chews rather than sucks its prey, using the fangs to grip and press food against a line of smaller teeth on the front edge of each chelicera.

Chelicera showing fang, smaller teeth and flat inner surface.

The inner edges of the chelicerae are flat and relatively hairless as they fit closely against each other, with fringing hairs around the edge as shown above (these may be used to stridulate i.e. produce sound when rubbed together, at least in some species). Looking inside a chelicera, there is evidence of the mobility of this structure required in order to chew and manipulate food - connective tissues can be seen which would have been attached to muscles that move the chelicera and fang.

View into a chelicera showing the bases of small teeth, plus the remnants of connective tissue used to move the appendage and fang.

That brings me to the end of my trilogy of tarantula anatomy posts. There are of course many other structures in a live spider, but the skin is easy to manipulate and can be dissected without harming the spider which has left it behind. So, if you are a tarantula-keeper, why not have a look the next time your spider moults - there are many interesting structures the closer you look.

Tarantula anatomy II: the cephalothorax

My previous post looked at the abdominal structures seen in the shed skin of a tarantula - this time I'm moving forward a segment to look at the cephalothorax i.e. the fused head and thorax.

The thorax with the top removed

With the upper surface removed, the empty thorax shows a clear pattern - the central section (the inner surface of the sternum) surrounded by the eight cavities associated with the coxa (1st segment) of each leg. The thorax is of course empty as the spider has emerged, but a slightly different view shows how complex this must be.

The empty thorax looking forward into the chelicerae

Here the structure is clearer still - the sternum at the base, the eight cavities through which the legs will have been extracted, plus corresponding cavities at the front (top of the photo) where the chelicerae (fang-bearring jaws) and head were pulled free. The white tuft of hairs between the chelicerae remains as do some white thread-like connective tissues within the empty legs. The sternum also clearly shows small holes such as those where book-lungs were joined to the outosde air via spiracles (breathing pores). Looking inside the upper surface of the cephalothorax, further structures can be seen.

Inner surface of the cephalothorax showing sculpturing

Close-up of sculpturing within the cephalothorax

 The inner surface shows both vaulting associated with the muscle attachments of legs and chelicerae, forming a central point between the attachments of these appendages. The lower photo is a closer look and shows finer mesh-like sculpturing presumably associated with the muscle attachments and possibly the passage of bodily fluids around them and/or the locations of nerves such as those serving sensory bristles. Near the front edge, the remnants of the eyes are also visible as their surface is also part of the exoskeleton and hence moulted along with other structures. Compared to the overall size of the spider, the eyes are small and closely clustered, possibly indicating the importance of, for example, sensory bristles to an animal that spends much time in a dark burrow during the day.

Front view of the cephalothorax showing eyes

Closer view of the inside surface of the cephalothorax showing how the eyes protrude outwards.

The upper surface of the cephalothorax - this back-lit view shows that there are eight eyes arranged on a small dome.

So, although the spider has left, certain structures - as with the abdomen - remain; the third and final part will look at appendages, including legs and fangs!

Tarantula anatomy I: the abdomen

A few days ago, I was chatting with a friend of a friend on facebook after they posted a picture of the moulted skin of their pet Mexican Redknee Tarantula Brachypelma smithi. The upshot was that they offered to send me the skin so I could have a look at it under the microscope and see what interesting features were visible. It turns out that there were quite a lot - more than can fit into a single post - so here is part 1, looking at abdominal features.

Dorsal view of the tarantula skin as it arrived in the post

The skin was well packaged and in really good condition and shows first of all how the spider moults. The top of the abdomen and cephalothorax split and peel back as a long flap, and the spider emerges up and backwards, pulling its legs and other appendages free. The new exoskeleton - including the fangs - is soft and needs to harden, and hence the spider will not be able to feed for a couple of days after moulting.

Looking at the abdomen, there are the familiar long bristles that you might expect to see, but a closer view (and indeed touch) shows that the texture is actually very different. There is a dense covering of shorter, softer hairs which look and feel much like moleskin - quite unexpected if you don't know what to expect!

The soft hairs of the abdomen along with longer, coarser bristles.

The long bristles are important as they have a sensory function, whicle some others form an important defence mechanism, being brushed off towards potential predators using the legs. These 'urticating' hairs (the paler patch top left in the photo above) are much smaller but are barbed and cause irritation to areas such as the eyes. In the wild, the spider would spend most of its time in a burrow in an earth bank and use these hairs to deter predators such as coatis - though large and fearsome-looking, these spiders rarely bite and have only weak venom. In this genus, the urticating hairs are classified as Type III (there are six recognised types) which are 0.3–1.2 mm long and particularly irritating to mammals, including humans who sometimes develop a rash as an allergic reaction. The biochemistry of the hairs is poorly known - they appear to be chitnous and are certainly not made of living tissue - their irritating effects have been assumed to be physical (i.e. the effect of having barbed hairs stuck in your eyes/skin), but there may also be direct chemical effects, at least in some species.

The bases of two sensory bristles showing the attachment points that fit into 'sockets' in the exoskeleton. Mag x40

The fine hairs on the surface of a bristle. mag x100

Some of the small, defensive urticating hairs - note the thin attachment points and covering of barbs. Mag x40

Close-up of the barbs covering urticating hairs. Mag x100

The last feature I want to look at here are the book lungs, a series of flat membranes (lamellae) that spiders use for breathing via a pair of pores (spiracles) and which increase the surface area for gas exchange in much the same way as alveoli do in our lungs. They are named after their overall form which is similar to a stack of pages in a book. In this specimen, initially they were deflated and looked like quite unremarkable white masses, but when teased apart, some of the fine structure could be seen with the lamellae attached to branches leading to the spiracle and thus the outside air.

A deflated book lung.

Close-up of the book lung showing individual lamellae.

So, just a few abdominal features here - after all, the spider took its other organs with it! However, they are still interesting and there's more to come as I will be writing about the cephalothorax and appendages soon...

The sticky world of sap-runs

It's well known that dead and decaying wood forms essential habitat for a wide range of species invlved in decay processes, plus many more that use such sites to burrow and nest. However, there are some situations which can be looked at individually and seen as providing even more specialised conditions, sometimes relating to living tree tissue, but siimilar in concept to dead-wood habitats. These include coppice stools, old parkland pollards and deadwood (saproxylic) or epiphytic fungi themselves, plus the one I want to look at here - sap-runs.

Bracket fungi on standing deadwood

Sap runs are living areas on the trunk or branches where sap oozes out for all or part of the growing season (Fry & Lonsdale 1991). These occur on (probably) all tree species - some such as elms and horse-chestnuts more than others - and the one I saw recently was on a branch of an oak. At the time I was looking at, and taking photos of, the rarer Wild Service tree in front of it. However, I noticed a flurry of invertebrate activity on a partly dead branch and initially thought it was a hornet (Vespa carbro) nest as I could see these large, and increasingly rare, wasps darting in and out of crevices and crawling on the bark.

Wild Service tree with the oak behind. The red arrow indicates the branch with a sap-run.

There may indeed have been a hornet nest, but what held my attention further was the behaviour of not only the hornets but also a host of other insects including various flies and Red Admiral (Vanessa atalanta) butterflies. The photos below illustrate some of this (apologies for the grainy images - it was the best my zoom could manage!)

A hornet on the underside of the branch

The most striking behaviour was the amount of aggression between the different species; initially hornets chasing butterflies and flies away, but also butterflies tussling with each other. In itself, this isn't unusual - male Red Admirals are highly territorial, but this happens in spring - the behaviour seen here occurred in late summer so is clearly something else.

Three Red Admirals fighting above while one feeds on the underside. Also note the large flies on the upper surface which may be feeding or basking.

This photo shows the aggression fairly clearly, but more importantly the position of the butterfly below. This is the exact spot occupied by the hornet above and was where any insect that avoided the fighting landed. Clearly there was a valuable resource here - and the only one that seemed plausible was a small sap-run. With sap-runs an important source of nutrient-rich fluid, it is unsurprising that individual insects were competing for it, especially if the run itself was tiny as seems to be the case here. The flies were unidentifiable at a distance and smaller species may well have been present - in fact this is highly likely as sap-runs are known to support specialists such as the gnat-like flies Mycetobia spp. and Sylvicola cinctus, midges Forcipomyia spp., hoverflies Brachyopa spp. (whose males hover near sap-runs and attempt to mate with any females that land on them) and Ferdinandea cuprea, and the beetles Glischrochilus hortensis, Cryptarcha strigata and Epuraea aestiva.

These are only a few examples and there are many others, especially among the hoverflies as the numerous mentions of sap-runs by Rotheray & Gilbert (2011) can testify, including the danger of becoming trapped in sticky fluid that eventually becomes amber. As noted by Kirby (2001), this highlights the importance of sap-runs, along with many other features of trees (rot-holes, dead branches, ivy) that are sometimes removed as signs of 'ill-health', such as by being selectively removed during woodland thinning rather than being selectively retained. In fact, it seems clear that larger and longer-lasting sap-runs support more diverse species assemblages and so trees with large, deep injuries forming sap-runs should be retained just like those with large dead-wood features. It means overturning some of the received (but erroneous) wisdom ingrained in aspects of woodland management, but structural diversity is of key importance and the countryside isn't meant to be neat!

References

Fry, R. & Lonsdale, D. (eds) (1991). Habitat Conservation for Insects - A Neglected Green Issue. AES, Middlesex.
Kirby, P. (2001). Habitat Management for Invertebrates: A Practical Handbook. RSPB, Sandy.
Rotheray, G.E. & Gilbert, F. (2011). The Natural History of Hoverflies. Forrest Text, Tresaith.

Soldiering on with identification

I was down on our local community farm the other day and took the opportunity to have a look at the large pond which is used for both wildlife and irrigation. Aside from some familiar dragonflies and water-bugs, I noticed something a little odder twisting and turning in the water. Around 50mm long and spindle-shaped, it was certainly a larva of some sort, but what type?

Mystery larva, around 50mm long

Firstly, it seemed clearly aquatic - though not an efficient swimmer like, say, water beetle larvae, it was making slow progress and was not acting like a terrestrial larva that had fallen in. It's overall form is that of a fly (Diptera) larva, but a closer look is needed to narrow it down further. Being aquatic, dorso-ventrally flattened, large, leathery and having an elongate, blunt-ended last abdominal segment (bottom right here, indicated by a red bar), it is relatively straightforward to determine that it is a soldier-fly (Stratiomyidae) of the genus Stratiomys. which feeds on microscopic organisms (Smith 1989). However, identifying it to species is not so easy.

In the next photo, the ring or tuft of specialised floating hairs is just visible (the thin white structure indicated by a black arrow), and is used to keep the last abdominal segment with its spiracle (breathing hole) close to the surface. Indeed, as the larva moved, the tuft was always at or near the surface. Some of the abdominal segments have small pale triangular tooth-like projections at the front corners (indicated by white arrows), while the head (red arrow) bears yellowish bristles on the underside. The lack of ventral hooks beneath the hind edges of segments 9 & 10 separate it from Odontomyia ornata even though the pale side stripes make it appear superficially very similar.

Stratiomys sp. larva - oblique dorsal view.

Stratiomys sp. larva - lateral view.

The abdominal features are important for identifying different larval Stratiomys, but using the key in Stubbs & Drake (2001) presents a problem as the authors note that there are no known features which are reliable for British specimens. The abdominal projections indicate either S. longicornis or S. singularior but both of these are associated with brackish waters and this is a freshwater pond. However, whereas S. longicornis specialises in saline conditions, S. singularior is occasionally known from waters which are not saline, but which have some similar water quality characteristics such as brick-pits. This is not a brick-pit, but it does have edges made largely of sandy gravels, some of which is spoil from elsewhere, so the water may be suitable for this species. Also, the large size of the larva is another feature in its favour (it can be up to 65mm long). So, although the habitat suggests S. potamida or the rare S. chamaeleon, the abdominal projections discount these and hence I have tentatively identified this as S. singularior, albeit in a slightly unusual location.

References

Smith, K.G.V. (1989). An introduction to the immature stages of British flies. Diptera larvae with notes on eggs, puparia and pupae. Handbooks for the Identification of British Insects 10(14): 1-280.
Stubbs, A. & Drake, M. (2001). British Soldierflies and Their Allies. BENHS, Hurst.