Into the World of the Darkling Beetle

Silkworm Moth (Bombyx mori)

Silkworm Moth (Bombyx mori)

As seen in some of my previous posts, one of my interests is in entomology. I find insects, (and arachnids, too), to be fascinating, intelligent, and resourceful creatures. (On a strictly non-professional level, I also think they’re as adorable as other animals). As I have said many times, I am the person running toward the insect or arachnid that other people are fleeing from. And every year, I excitedly await the Bug Fair. It’s an annual event at the Natural History Museum of Los Angeles, and this year it is next weekend, May 16th-17th.

Painted Lady Butterfly (Vanessa cardui)

Painted Lady Butterfly (Vanessa cardui)

In the past, I have kept and studied different types of insects. When I was in 4th grade, I had a praying mantis, and later on, a silkworm that later became a silkworm moth (Bombyx mori). I acquired both of these from my teacher who gave one praying mantis, and later, one silkworm, to each student in the class, as part of our study of insects, and in the case of the moths, metamorphosis. Also when I was a little older, I ordered a butterfly kit. Five caterpillars came in a little container. This container has all of the food and everything that the caterpillars need in order to survive, so no action is needed until they pupate. The kit included a thin easy-to-assemble box. Once all five caterpillars had climbed to the lid of the container and became chrysalides, they were carefully transferred to the box, where they would later hatch into Painted Lady butterflies(Vanessa cardui). Plus, I had a number of insects and arachnids that I collected from my backyard when I wasyounger, and a couple of ant farms.

My current passion however, has become Tenebrio molitor–Darkling Beetles.

Beetles in general have always been one of my favorite insects to research and study. I spent a few months last summer studying Green June Beetles (Cotinus mutabalis), and at the end of last year, I joined the Coleoptorists Society.

Green June Beetle (Cotinus mutabulis)

Green June Beetle (Cotinus mutabulis)

Darkling Beetles, are also called Mealworm Beetles, because their larval form is the mealworm. Mealworms can be bought in any pet store, where I acquired my first batch, as they are often used as live food for a variety of reptiles. I bought a 50-count container of mealworms and put them into a jar that I had prepared for them. I had done research on habitats for mealworms and for the beetles. So I had a layer of dry oatmeal, under a layer of dry bread crumbs mixed with flour, under yet another layer of oatmeal. For the first fifty mealworm babies, this was fine. But once the mealies pupated and the beetles began emerging, these beetles of course multiplied, which is what I wanted anyhow. However, the jar was now too small for the last of the mealies who had yet to pupate, plus the current pupae, and the adult beetles, that were laying eggs, hatching more mealworms. So I bought one of those Critter Keepers, those plastic containers with a ventilated lid that has a little trap door in the top. I transferred the beetles into this, with the same oatmeal/breadcrumbs-flour/oatmeal combination lining the bottom.

In back, left to right, the original jar, now housing pupae, the container for tiny mealies and injured beetles, and then mealworm habitat. In front, circular beetle habitat.

In back, left to right, the original jar, now housing pupae, the container for tiny mealies and injured beetles, and then the mealworm habitat. In front, circular beetle habitat.

Since then, I am now on my second/third generation of beetles, and my third/fourth generation of mealworms; it’s not that I don’t know which generation I am currently on, it’s that as they multiply, generations tend to overlap, so I concurrently have two different generations of both beetles and mealworms.

I have three to four ongoing habitats at any given time. The Critter Keeper that I bought for the beetles, now houses the mealworms; the beetles were relocated to a larger, circular keeper. The original jar is used for the pupae, and at this time, I have several baby mealworms, which are practically microscopic in a smaller version of the mealworms’ habitat. As they get larger, I will put them in with the other mealworms. I think there are some beetle eggs in with the tiny mealies as well. I also find some beetles that emerge deformed or somehow injured, and these also go into this habitat, as I’m not sure if putting them in with the healthy beetles would be good for them.

They do need to eat of course, and to be provided with moisture. This is accomplished by a slice of apple replaced every day or two. Carrots and potatoes are also suggested, but I haven’t tried those yet. My beetles and mealies seem to love apple, so that works for now.

Some of my mealies in their mealworm habitat with the dry oatmeal, and pieces of lettuce.

Some of my mealies in their mealworm habitat with the dry oatmeal, and pieces of lettuce.

I like to experiment with vegetables as well, to see if they have any preference. It turns out that they like the red, romaine, and iceburg versions of lettuce, but they like it more if I run the lettuce under water first, and give it to them while it’s still moist. And it just so happens that they like broccoli as well. With the first generation of beetles, I tried tomatoes and raspberries; they didn’t like those, and even seemed to be wary of it. My current beetles were given a tiny portion of cilantro, just to see how they reacted. In case you’ve never tried it, cilantro has a very strong and distinctive flavor to it. I like it, but I also know a few people who don’t. I figured it was too strong for the beetles, as they tend to like blander foods. The verdict: they did not like it! (As I kind of surmised beforehand). The few beetles that actually approached the cilantro, ran away from it after touching it with their antennae; I don’t think any of them actually ate any. I immediately removed the cilantro, and gave them their daily apple slices instead. They seemed grateful for that; maybe they wanted to get rid of the cilantro taste. The mealworms also eat the apple and lettuce; they will eat the oatmeal, too.

Four of my pupae in varying colors as they are in different stages of development.

Four of my pupae in varying colors as they are in different stages of development.

The mealworms begin at a practically microscopic size, hatching from microscopic eggs. They are a light brown color, though I have seen a couple of white ones, and they eventually grow bigger, ultimately reaching a little over an inch-and-a-half long, molting as they grow. Their last molt is into a pupa. The pupae are white at first, and gradually change to a brown color, getting darker as the beetle is getting ready to emerge. The adult beetles are white when they first emerge, and over the next day or two, go to a golden brown, then a light brown, to a dark reddish-brown, before becoming the solid black color. Hence the name Darkling Beetles.

Beetle behaviors that I have observed:

The first generation made several little solidified…I call them balls, but they weren’t necessarily always spherical. It appeared to be made out of the oatmeal and bread crumbs, but I’m not sure what they were, nor what purpose they served. Similarly, my current beetles have made a few “balls” solely out of oatmeal. Again, not sure why. They are solid and don’t fall apart, even when I pick them up. Any answer is welcome, so if there are any entomologists or other enthusiasts out there who have witnessed this same behavior, and/or know why the beetles do this, I would love to hear about it. Likewise, should I figure out this behavior, I will record it here.

Darkling Beetle (Tenebrio molitor)

Darkling Beetle (Tenebrio molitor)

The beetles tend to group together. And lately I have noticed this in the mealworms, too. The mealworms do this is one, sometimes two, corners of their habitat; the beetles can be found in two to three groups, always along the edges of the habitat. Again, I don’t know why they do this, nor do I have any way of telling if the same beetles are always the same ones that gather together.

I witnessed, only once, when one of the beetles fell over on his back, another beetle actually helped him and flipped him back over onto his feet. Though I have also noticed that they are always able to eventually flip themselves back over on their own. I will also flip them back over myself if I notice any of them on their backs before they get back to their feet themselves.

One of the older Darkling Beetles, (partially hidden), and a newer, light-colored Darkling Beetle.

One of the older Darkling Beetles, (partially hidden), and a newer, light-colored Darkling Beetle.

Something that just happened today: a beetle that had newly-emerged from a pupa, had the pupa skin stuck to a couple of his legs. This has happened before, and I can usually pull it off, but I have to be very gentle and very careful, as the beetles’ legs are very delicate, and I do not want to inadvertently pull the leg off instead. Today, I only got it off of one of the feet, but couldn’t get it off of the other one, and was afraid of trying too hard and hurting the beetle. I put him in the habitat with the other beetles, thinking that if he walked around on the oatmeal or went under it, the skin may eventually rub off. But I saw a couple of other beetles approach this one, and they were around the pupa skin that was caught on the beetle’s leg. I don’t know if they saw the problem and were trying to help the other beetle, or perhaps they were eating the pupa skin, I even observed it close up, but couldn’t tell exactly what was going on. Either way, they got the skin off.

They seemed to recognize me after awhile. I have heard that honeybees can memorize faces, and perhaps they are not the only insects who can. I do see the beetles and feed them on a daily basis. So they may at least know me as the person who feeds them, and occasionally picks them up and studies them.

I have been recording my observations, hypotheses, and research in a log book. I found one online that is black in color, hardback bound, and simply says “Beetles” on the front of it. Raising Darkling Beetles, and watching them from mealworms, to pupae, to adults, is very interesting, and helps me to learn even more about beetles, and entomology in general. I have even had the benefit of seeing a mealworm become a pupa, and a beetle emerge from a pupa; I was able to record video of these events as well.

A few of the beetles gathered around lettuce.

A few of the beetles gathered around lettuce.

And as long as the beetles keep reproducing and thriving, I will keep raising them, learning even more as I go along.

And I don’t plan on stopping with beetles, either. There are other insects and arachnids out there that I would like to go to next. There are Islander Roaches for one; these too, can be bought at a pet store, sold as food for reptiles like the mealworms are. And eventually, I plan on buying tarantula as well.

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