When stick insects become lizards

 A long, long time ago in a galaxy far, far away, I accidentally tore off one of my stick insect's limbs.

Peggy II not long after the incident

Though this was a complete accident, I have felt incredibly guilty ever since, as I feel it could have been avoided had I been more careful. It was an enclosure cleaning day, so I was doing what I normally do and cleaning out the privet stalks from the cups of water - only I forgot to check if there was a stick insect on the back. Hence, I had hold of its leg more than a light touch, causing it to freak out and drop as if I were a predator. I didn't even notice at first, thinking it was a branch and throwing it to the floor (I cover my floor with puppy pads for easy cleanup). Until I looked again and thought, "Huh, that looks an awful lot like a leg." And it was. I felt so bad because it was so obviously my fault as the owner. But then I thought, "Oh, no worries! It should grow back."

I'm not sure how many people know this, but like lizards, stick insects can regenerate their limbs. So when Peggy II (the second, as in my last set of stick insects; I had one called Peggy, as I thought she lost a limb, but I was mistaken; Peggy II has actually lost a limb) hadn't grown her limb back after her next moult, I started to do a bit of research online as to why not.

Peggy II now

As it turns out, stick insects actually need 3 moults to have a fully regenerated limb, and since Peggy II was nearly at adulthood (as it turns out, 1 moult away), she would never regrow her limb. She has five others, and so she will be able to live fairly normally. I can still imagine that she has some drawbacks because it was her right front leg. Whilst researching, I also wondered, "Why do stick insects have limb regeneration as a characteristic?" So, looking into it, I discovered that the most common way for them to lose a limb (at least in captivity) is due to low humidity during moulting. Low humidity means that the old skin dries too quickly, meaning the new, longer limb is trapped within the dry old skin and is therefore less flexible. Stick insects are required to moult as, unlike a lot of animals, their skin doesn't grow with them as they mature, so they have to shed it. For the type of stick insect I have (Carausius morosus or Indian stick insect), they take 6 moults to mature.

Some other ways stick insects can lose their limbs are:

• Fungal infections, where you would have to entirely deep clean their enclosure and then quarantine some of them, if necessary.
• Overcrowding, which can cause them to start biting others or knock each other's legs off. The general rule for a stick insect enclosure is, at a minimum, it must be three times as tall as the stick insect’s length and at least twice as wide as their length. Otherwise, they have insufficient room to moult.
• This final reason is, I believe, why stick insects evolved to have this trait in the first place: predation. If a bird or some other sort of predator were to take hold of a leg or an antenna, the stick insect could drop the limb and run away.

image from this study

Whilst this may seem like an ideal thing for stick insects and other animals which can regenerate limbs, it comes with tradeoffs - which is another thing I learnt while researching this topic. For example, in some lizards, if they were to regenerate a lost tail, it can come with the side effect of negatively affecting their ability to produce offspring. I think this would make sense as, obviously, regenerating a limb costs the body more energy - think of when you are ill. You're generally more tired because your immune system is using lots of energy to fight the disease, and regenerating a limb is likely a lot more taxing on the body than fighting an illness. I even found a study which investigates whether or not a stick insect having to regenerate a limb early in its life will affect its overall wing growth. In this experiment, Sipyloidea sipylus (also known as the pink-winged stick insect or Madagascan stick insect) was used - this species has wings to assist them in slowing their fall from tall trees, whether to escape predators or to seek mates. The results found that those which had to regenerate a limb were not only smaller in length compared to those that didn't, but they also had a 15% decrease in wing area and during their simulated flight tests, they consistently fell much faster. If you're interested, click here to read the study.

Although the sample size was small for this study, I believe it shows the right idea, and it would make sense for it to be true. Regenerating a limb is going to require an insane amount of energy; imagine how much extra food they'd have to consume. Imagine if humans could regrow limbs! Stick insects and lizards are only small compared to humans, so imagine how much energy we'd require.

I hope this blog post was coherent enough and made sense; I really enjoyed researching this, and I thought the study was such an interesting read! Though I'll be honest that when I read about them removing limbs with forceps and then euthanising the stick insects when they were grown upset me quite a bit. I know it was a part of the study, and I don't know how else I expected them to remove the limbs, but it still upset me.