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Leopard slugs (Limax maximus) are large (4 to 5 inches long)! Native to Europe, they arrived in this country in the 1800s, probably hitching a ride in potted plants. Observers noted the first in the United States in Philadelphia in the 1860s and discovered the first in Massachusetts – in Springfield – in 1881. Since slugs do not travel very far on their own, the fact that leopard slugs today inhabit every continent except Antarctica is a testament to human trade: Slugs populate new areas when people move them to new locations in potted plants, produce, wooden boxes or pallets, farm equipment, compost and so on.
Leopard slugs start out as males but develop female reproductive organs as they mature. Since adult leopard slugs are both male and female, any one can fertilize the eggs of any other one. Mating with other slugs increases genetic diversity, so leopard slugs do not fertilize themselves except perhaps under extreme conditions.
When the hours of daylight begin to increase in the spring or decrease in the fall, leopard slugs realize it is time to reproduce. One lays a pheromone-laced slime trail indicating that it is ready to mate; an interested slug follows the trail, and the two chase each other at the breakneck speed of six inches per minute. Most often the two crawl out onto a tree branch and wrap themselves around each other, each secreting a ropelike thread of mucus. The two threads intertwine as the pair twists counterclockwise. Then the pair drops down on the thread, dangling in midair. (You have probably never witnessed this bizarre phenomenon because the spectacle occurs at night.)
Out of the right side of their heads, they stick out a long light blue phallic organ (a penis, if you will). This organ is half as long as their body. They intertwine these organs to exchange seminal fluid. This fluid may be used to fertilize eggs, or it may be eaten. (Slugs can store sperm, so they don’t necessarily need this sperm to fertilize their eggs.) After mating, one slug may drop to the ground while the other returns to the branch by eating the slime rope. Each snail then crawls away to a dark, damp spot to deposit hundreds of eggs. The eggs take about a month to hatch. The tiny babies that emerge reach maturity in two years (although most are eaten by predators before they become adults).
You might think slugs are nothing but snails without shells, but this would not be true. Since slugs evolved from snails, leopard slugs still have a small internal shell, about the size and shape of a fingernail. Without a home on their back, slugs are faster and more mobile than snails. They can wedge themselves into places snails cannot. They also lose the protection a shell provides, and since their bodies are mostly water, they dry up easily.
To keep from drying out, slugs produce slippery mucus that coats their body. The mucus has other purposes too: It lubricates the ground over which they travel and it foils birds when they try to get a good grip. Slugs also communicate through chemicals in their slime, and they follow their mucus trails to find their way back home.
Slugs produce more than one type of mucus. One type is thin, watery and quite slippery, which helps them move over rough surfaces. Another type is thick and sticky, which helps them adhere when they crawl up walls.
The underside of a slug is called the foot. Its wavelike muscular contractions thrust a slug forward, leaving behind a mucus trail. When a slug travels on a smooth surface such as plastic, glass or leaves, the entire foot sticks to the surface and leave a continuous trail; when a slug travels over an uneven surface, such as wood or bumpy ground, it arches its body and only partly sticks to the ground. That leaves a broken mucus trail. Next time you see a slug’s trail, look to see which kind it is to know how the slug moved its body.
Leopard slugs have a tonguelike organ with 150 rows of microscopic, backward pointing teeth. Slugs use this organ to scrape holes in plants – and to battle other slugs.
Slugs have two pairs of tentacles. The longer pair has eyespots that sense light and dark. The shorter pair senses odors. (On the head behind the right tentacle is an orifice or genital pore where the penis extends when mating.)
Leopard slugs like urban environments. They are often found in gardens, abandoned buildings, basements, cemeteries, parks, greenhouses or stacks of firewood. Because this slug is so large, it is easily spotted in your garden and often blamed for any damage to the green plants there – but these feed mostly on fungi, mushrooms and their roots (mycelia). It is other, smaller slugs that feast on your lettuces. Leopard slugs also eat lichens, decaying leaves, roots, flowers, seeds, feces, dog and cat food and carrion. In damp areas, such as your cellar, leopard slugs might feed on moldy biofilms on wallpaper, old books or documents.
Green plants, however, are not typically a part of the diet.
These slugs have been described as cannibalistic, probably because people have seen them biting each other. They do it to lay claim to the best shelter when it becomes scarce – and because these slugs are so large, it can be difficult for them to find a good damp place to shelter.
This is probably more than you ever wanted to know about leopard slugs, but I will throw out one last fact: During hot summer weather these slugs find a cool, damp place to estivate, or enter a state of dormancy in response to heat. During cold winter weather, they hibernate, also a state of dormancy but in response to cold temperatures.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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👍🏼 Ms. Farley, your articles are treasures!