Millipedes, Centipedes and Sowbugs

Millipedes. The millipedes are elongated, worm-like animals with 30 or more pairs of legs with most body segments bearing two pair. Some of the larger species have as many as 400 legs (Figure 2A). Their bodies can be either cylindrical or flat; the antennae are short and seven-segmented. One of the more spectacular species is a twelve-inch African millipede that for years has been sold in pet shops in the United States. Unfortunately this practice was recently discontinued due to governmental regulations. It is quite interesting to watch these bigger millipedes move.  With almost 400 legs they need a system to keep from tripping themselves.  When a millipede “runs,” it lifts each pair in sequence with a wave-like progression of legs moving down the body.

Figure 2A. Twelve-inch African millipede.

If disturbed, millipedes will typically coil up with their head (most vulnerable part of the body) in the center (Figure 2B).  There are a group of millipedes called the pill-millipedes which roll up much like a pillbug or so called roly-poly.  These occur in many tropical areas with a species from Madagascar reaching the size of a golf ball.          


                                                                                              Figure 2B.  A rolled up African millipede.  Right.  A pill millipede.

Millipedes typically feed on decaying vegetation, although occasionally they become pests on plant parts with ground contact. Most millipedes in the continental U.S. rarely exceed three to four inches in length; however, as indicated, some of the tropical species can reach one foot in length. Sometimes millipedes, occasionally accompanied by centipedes and sowbugs, will migrate in great numbers.  This is believed to be the result of a heavy buildup because of extremely favorable environmental conditions followed by drought.  Migrations at times have been so heavy as to make it necessary to sprinkle sand on slippery railroad tracks to provide traction for the driving wheels of the trains.  In one migration a farmer shoveled several gallons of these critters a day from his front porch for three weeks.

These animals are relatively harmless to humans and do not bite; however, a few species can defend themselves by ejecting irritating fluids a distance of several inches. A Haitian species is reported to discharge its secretion at distances close to a yard. The fluid is dangerous to the eyes and is responsible for occasional blindness in chickens. One of the irritating fluids in some millipedes is reportedly hydrogen cyanide, the same chemical used in the gas chamber.  Small amounts are released and it is estimated the total is approximately 1 % the amount needed to kill a human but 40% the amount needed to kill a sparrow.

Closer to home an insect dealer at one of our insect fairs was recently observing a small tropical species of millipede (about four inches in length) when it shot a spray of some unknown chemical from a distance of over two feet directly into his eye.  The result was quite painful and resulted in considerable swelling (Figure 2C).

Figure 2C. A swollen and inflamed eye due to contact with millipede defensive chemical.

Millipedes grow slowly and some can live for several years. Most species prefer damp habitats and remain hidden during the day under objects.

Centipedes. Centipedes are elongated, flattened, worm-like animals with 15 or more sets of legs with each body segment bearing a single pair (Figure 2D). The last pair is usually longer and pointed directly backwards and looks very similar to the elongated antennae. As a result, the head end of many centipedes looks almost exactly like the tail end. This is advantageous since a potential predator may grab the wrong end which could result in a painful and potentially fatal mistake. Good reason to be left alone! The first pair of legs is modified into a pair of fang-like poison jaws that, in larger species, are capable of inflicting a painful bite (Figure 2E). Smaller centipedes are considered harmless to humans, but the bites from larger species can be quite painful, resulting in local swelling, and which can debilitate an adult human for a few days. Some of the tropical species are quite large and are greatly feared by humans.

One of the most notorious of these night hunters is a giant that live in caves in Venezuela and can reach a length of 18 inches.  This powerful beast hunts bats.  It frequently can be seen crawling over cave ceilings.  It will typically pass up smaller prey such as cockroaches on it way.  Once seize with its venomous poison jaws the bats are said to die in as little as 30 seconds. Although probably not capable of killing a human, villagers in Venezuela claim that the bite of one did kill a child.

At times it almost appears that these and other arthropods have the ability to think.  Of course this is quite doubtful, but they have been well equipped to react instinctively (almost appearing intelligently) too many circumstances.  We recently watched a large Vietnamese centipede catch and feed on a cricket.  Once caught the centipede immediately cut off the cricket’s legs thus removing its ability to escape.  Thinking that this was an accident we fed it another cricket with the same results.  Taking this impromptu experiment a step further we fed a cricket to a different centipede with the same result.

Figure 2D.  Twelve-inch Vietnamese centipede.



Figure 2E. Underside of centipede head showing poison jaws.

The authors recently documented the case of a bite from a large species of Arizona centipede. In this situation, a homeowner bent over to drink from a water hose. Unfortunately, a 12-inch centipede had recently occupied the hose and, when the water was turned on the centipede appeared and bit the individual on the tongue. His tongue and mouth swelled considerably and he was in pain for several days.

These organisms are nocturnal and remain hidden during the day in protected locations, such as under loose bark and rotting logs and in tunnels in the soil.  Centipedes are fast moving and predaceous on spiders, insects and other small animals.  The diet of the larger species is not limited to insects or other arthropods. They certainly are large and strong enough to subdue small snakes, mice, lizards and even birds if they can catch them. They can easily be distinguished from millipedes by how quickly they move. Because millipedes feed on plant tissue, there is no need for agility and they move quite slowly. You don’t need to be fast to run down a leaf. Generally, plant-feeding arthropods are slow while predators are fast.

The common house centipede is about two to three inches long, grayish-tan in color with long antennae and extremely long legs (Figure 2F). As the name implies, this creature is commonly found running over walls and ceilings in homes. These are harmless and could be considered beneficial as they feed on cockroaches, moths, houseflies, spiders, and other pests found in and around the home. However, I am sure most homeowners would not consider them an added benefit to the home, especially the Thai species shown below.



Figure 2F. Left. Common US Species. Right-Thai house centipede-Image courtesy John Moore.

With the exception of the common house centipede, centipedes are not as commonly found in the home as millipedes and, when encountered, never in the numbers that can occur with millipedes.  Of course part of the reason for this is that their outdoor populations never reach that of millipedes.  However, if a centipede does occur indoors it can be somewhat stressful.  One evening (on a Hawaiian trip) one of our visitors was washing dishes and a seven inch centipede appeared from under the dishwashers (apparently a fairly common occurrence in Hawaii).  She came running from the kitchen screaming centipede, centipede!  According to her she couldn’t sleep that night. A note of importance!  Never try to transport living centipedes on a commercial airline. After capturing the centipede I decided it would make a nice addition to our insect zoo at Cal Poly.  I neatly packed it away in a small container in my luggage.  Of course on getting home the critter was dead.  I guess it gets pretty cold in the cargo part of the plane.

On a later occasion I decided to bring back several giant millipedes (12-inch beauties) I had collected in Malaysia.  Keeping in mind that they wouldn’t make it in the cargo area I decided to carry them in my carry-on luggage.  While waiting for the plane to begin loading I decided to check and see how they were doing—big mistake!  One of the stewardesses saw them out of the corner of her eye and freaked.  She first thought they were snakes and called security.  After convincing all involved that they weren’t snakes, I still couldn’t convince anyone that they were harmless.  I was subsequently escorted from the airport to release the monsters back into nature-almost missed that flight.  I guess it could have been worse.  Luckily for us, the movie “Snakes on a Plane” had not yet been released.

Not learning from those experiences I later transported a large variety of living critters (scorpions, walking sticks, millipedes, tarantulas, etc.) on Alaskan Airlines up to Oregon for a presentation I was giving to the Oregon Pest Control Association.  I had no problem getting them on the plane on the way up, but was stopped by security on the way back.  Major alert-you would have thought I was some kind of terrorist.  After convincing the authorities that these critters were not going to kill anyone and that I wasn’t totally nuts, the additional problem was that it is prohibitive to bring living animals of any kind on a plane.  I think the exceptions were dogs and cats in cargo and goldfish.  They again wanted me to take the specimen out of the airport and release them.  After some fast taking and actually doing a short presentation on these arthropods to the airline officials, they let me put them in cargo after they triple boxed them. Again, we almost missed that flight as well.

We once stopped at a roadside stand in Thailand to buy some dried-roasted caterpillars for an afternoon snack (when in Rome do as the Romans) and were offered  a quart bottle of home-brewed whiskey; but instead of having a worm in the bottle, as does Mexican tequila, there was a 14-inch pickled centipede. Drinking the whiskey enhanced by centipede was said to give the consumer great strength (or a least make the consumer think he had great strength until he sobered up). This belief is apparently common in several of the South-East Asian countries. By the way, the worm in the bottom of the tequila bottle is a caterpillar that feeds on the century plant from which tequila is derived.

Crustaceans. This is a very large group of arthropods that occupy diverse habitats, but the majority is aquatic, with many marine species. The more familiar forms include the crabs, lobsters and shrimp. While these forms are beyond the scope of this text, there are a few common terrestrial crustaceans that we encounter in our daily life, mainly the sowbugs and pillbugs (Figure 2G).  Both were introduced from Europe and are now distributed throughout the United States and are, without a doubt, two of, if not the most, common backyard arthropods. Although these critters have several minor structural differences, one easy way to distinguish one from the other is that pillbugs, as their name implies, are capable of rolling up into a tight pill-shaped ball and the sowbugs are not. Both types possess seven pair of legs and have a fairly short life cycle, with two or three generations produced per year. Each female has approximately 50 young.


Figure 2G. Sowbugs (larger specimens) and pillbugs.

Being crustaceans, sowbugs and pillbugs are not well adapted for a terrestrial way of life. Unlike insects, the exoskeleton is not coated with an outside waxy layer to reduce water evaporation from the body. Similarly, the external openings of their breathing system are not fitted with valve-like structures (spiracles in insects) which can be used to close off the system when not in immediate use. This lack results in an additional source of water loss from the body.  Dehydration or the loss of water from an arthropod's body is a continuous problem. Because these creatures are relatively small, they have a rather large surface-area-to-body volume ratio (e.g., large area to lose water from and relatively small area to store it internally). In order to prevent dehydration, sowbugs and pillbugs remain hidden during the day in moist situations, such as leaf litter and under rocks and logs, and forage for food at night. It is common to find large numbers huddled together in order to reduce their surface area and conserve moisture.

Sowbugs and pillbugs feed chiefly on decaying organic matter such as rotting leaves and twigs, dead animals and even feces. On occasion, they cause minor damage to plant roots, seedlings and ripe fruit in contact with the ground. In cases where they become pests, their presence can be reduced by removing breeding and hiding sites.