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Welcome to my blog

This is where I post various musings about wildlife and ecology, observations of interesting species (often invertebrates)
and bits of research that grab my attention. As well as blogging, I undertake professional ecological & wildlife surveys
covering invertebrates, plants, birds, reptiles, amphibians and some mammals, plus habitat assessment and management
advice
. I don't work on planning applications/for developers. The pages on the right will tell you more about my work,
main interests and key projects, and you can follow my academic work here.

Sunday, 22 January 2012

Observations of Macleay's Spectres I: early stages

Last summer, I was given an unusual birthday present - a 35mm film canister full of eggs of the Macleay's Spectre stick-insect Extatosoma tiaratum, sometimes known as the Giant Prickly stick-insect. These are found mainly in the Australian forests of Queensland and New South Wales where they feed on the foliage of eucalyptus trees, although in captivity they eat a range of unrelated species - one popular food is bramble (Rubus fruticosus agg.) which is what I use as it is readily available for free. However, young bramble leaves can contain noxious chemicals that make them unpalatable or even toxic, especially to young insects - I tend to remove these, and when some have been included by accident with more palatable older leaves, the insects ignored them.There is plenty of information about this species (e.g. from the Phasmid Study Group) as they are popular pets for those who like their companion animals to be six-legged, but I have made some observations which may be of interest. So, let's start at the beginning - with eggs...

Egg of E. tiaratum with the lid (operculum) open.
Close-up of the operculum showing the translucent sealing membrane around the edge. There is another membrane below this which seals the opening to the egg and which is broken to allow emergence by the first-instar nymph.
The eggs are a few mm long, oval with a sculptured ridge along one long axis, and a round lid (operculum). They are speckled with various shades of yellow and brown, camouflaging them to look like seeds when they fall to the forest floor in the wild. In those I have at home, the eggs are a pale yellow-brown when freshly laid and dry, but when placed on damp tissue to (eventually) hatchm they become a dark brown - presumably this aids camouflage with dry soil conditions also being paler and darkening when there is rain. When females lay eggs, they often flick them several feet by twitching their abdomens, presumably to aid dispersal  - the function of this might be to prevent a cluster all being eaten in one go by a predator. The eggs have a coating (made of lipids and other organic materials) which is edible to ants and which induces them to take the eggs to their colony and eat this coating. The eggs remain otherwise intact and the ants dump them on the colony's waste pile where they hatch. Thus the eggs are likely to become clustered following the initial dispersal during laying but will have spent much of their time in an ant colony where the chance of predation is much reduced. Luckily, the eggs can hatch without the attention of ants which means that those missed by foraging ants and of course those in captivity remain viable.

Unsurprisingly, given their birthplace, E. tiaratum hatchlings (forst-instar nymphs) are ant-mimics, specifically of the large, long-legged 'spider ants' in the genus Leptomyrmex.

Recently hatched nymph showing the orange head and black body providing its mimicry of Leptomyrmex ants.
The orange head and black body, as well as overall shape closely mimics that of ant species such as L. darlingtoni, L. erythrocephalus and L. rufipes and so benefit from appearing similar to these toxic species (the sting has degenerated in this group of ants and they instead secrete a chemical repellant).

When my specimens hatched, I couldn't help but notice that the nymphs were highly active (a well-known characteristic) and that their preferred direction was definitely up. This makes sense as they hatch on the ground but are foliage feeders and so need to climb quickly into trees to find food. After their first moult, they lose their ant-mimic coloration and develop one of a range of camouflage colours - oranges, yellows, browns, greys and greens - and they also become much less active, more like the typical behaviour of a stick-insect. They do make swaying movements to mimic the motion of leaves, especially when disturbed, but otherwise spend much of their time stationary or moving slowly as they feed, though being nocturnal much of their activity goes unobserved (or would if I wasn't so nosy). The image below shows three nymphs at different stages and with different colours. In subsequent parts of this series, I'll be looking at changes in colour and behaviour (in males and females, highlighting sexual dimorphism) plus the process of moulting.

Three nymphs showing different colours and stages of development. The two smaller ones are 2nd instar nymphs, the larger one a 3rd instar.

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