Delaware adopted the stonefly (order Plecoptera) as the official state macroinvertebrate in 2005. Delaware is the first state to designate a "macroinvertebrate" as a state symbol (Delaware also recognizes two other insect symbols: a state bug and a state butterfly).

The stonefly was chosen because it's an indicator of the excellent water quality in the state. It also provides a way to raise public awareness of water quality issues and complement citizen action programs such as Delaware Stream Watch.

Adoption of the stonefly as the official State macroinvertebrate was supported by the following schools: Gunning-Bedford Middle School, Salesianum High School, Delcastle Technical High School, Dickinson High School Environmental Club, The Independence School, Springer Middle School, St. Andrews School, and The Charter School of Wilmington (through participation in the Delaware Stream Watch Program).

Source:

Delaware Facts and Symbols: Delaware Official Website

Links:

State Insects - 50 States List