What is the Difference Between Sowbugs and Pillbugs?

 

What is the Difference Between Sowbugs and Pillbugs?

Pests… But Not Insects

Both sowbugs and pillbugs are part of the Arthropoda phylum, making them related to insects and arachnids, however, they are part of a different class altogether. They are members of the subphylum Crustacea, which range in size from microscopic plankton to the 4.3 m giant crab. Crustaceans include crabs, lobsters, krill, shrimp and, of course, sowbugs.  Due to the extensive diverse nature of this subphylum, it is broken up into five different classes: Branchiopoda, Ostracoda, Remipedia, Maxillopoda, and Malacostraca. Sowbugs and pillbugs are part of the largest of these classes, the Malacostracans which includes marine, freshwater and terrestrial invertebrates, making them more closely related to shrimp, crabs, and lobsters than any kind of insect. Within the Malacostracan class, they are organized into the order Isopod, which specifies that they are terrestrial-bound crustaceans.  

Biology and Appearance

Both of these terrestrial crustaceans are light to dark grey or brown in color, are oval shaped, range between 1/4 – 1/2 inch in length, have 7 pairs of legs, 2 antennas, and a hard-shell exterior made up of a line of plates. They are the only crustaceans that have evolved to live their lives fully on land, yet they still breathe through a kind gill on the underside of their abdomens. Female sowbugs and pillbugs also have a unique pouch on their undersides. Just like marsupials, they hold their developing eggs (usually in groups of up to 100) in these pouches and will even carry their young in them for 1-2 months after they’ve hatched. While, at a first glance, these “bugs” seem practically identical, and their names are even often used interchangeably, there are a few physical features that set them apart. Sowbugs tend to be flatter while pillbugs have a more rounded/cylindrical appearance. This rounded body shape along with the fact that the plates of pillbugs are not only overlapping, but also flexible, allows these little crustaceans to roll up into a ball when they feel threatened. This ability is known as ‘conglobulation,’ and earned them the common nickname: roly polys. Sowbugs lack this ability, but they do possess a pair of large uropods which are like a pair of tails protruding from their rears. Pillbugs also have uropods, but they are far smaller and typically only visible if they are flipped over on their backs.

Habitats and Diet

Sowbugs and pillsbugs both prefer dark and moist areas such as the undersides of logs, stones, piles of leaves, etc. Like most crustaceans, they are scavengers and will consume a variety of organic material both fresh, and decomposing. Typically, they are only pests in yards and gardens where they may feed on some of your vegetables or ornamental plants, but more often than not, they’re both rather negligible nuisances.    

Citations

Bousfield, E. and Conlan, K. (no date) Malacostracan – Crustacean, Encyclopedia Britannica. Available at: https://www.britannica.com/animal/malacostracan (Accessed: November 2020).

Phylum Arthropoda, Subphylum Crustacea (no date) Key to Australian Freshwater and Terrestrial Invertebrates. Available at: https://keys.lucidcentral.org/keys/v3/TFI/start%20key/key/Starting%20key/Media/HTML/Arthropoda%20Crustacea.html (Accessed: November 2020).

Potter, M. (2019) Sowbugs and Pillbugs, The University of Kentucky College of Agriculture, Food, and Environment. Entomology at the University of Kentucky. Available at: https://entomology.ca.uky.edu/ef439 (Accessed: June 23, 2021).

Ramsey, M. (2013) Holy Moly It’s A “Roly Poly” – Sowbugs and Pillbugs, University of California Real Dirt Blog. University of California Department of Agriculture and Natural Resources. Available at: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=12567 (Accessed: June 23, 2021).

Sowbugs, Pillbugs (no date) Texas A&M Agrilife Extension. Texas A&M University. Available at: https://texasinsects.tamu.edu/sowbugs-pillbugs/ (Accessed: June 23, 2021).

Taxonomy (2020) Basic Biology. Available at: https://basicbiology.net/biology-101/taxonomy (Accessed: November 2020).

Towne, J. (2019) Are Insects Animals?, Medium. Available at: https://medium.com/@jessicatowne/are-insects-animals-2f91da7506c (Accessed: November 2020).