The Malpighian tubules in female nymphs and adults of Pteronarcys californica Newport are larger and whiter than in males. This difference is detectable in nymphs as small as 300 mg in live weight (final instar nymphs weigh 800 to 1600 mg depending on sex), and is most pronounced in late nymphs and adults. These differences are thought to be related to the increased excretory load imposed by the synthesis of ooplasm in the late female nymph and adult.
Hathaway, Ralph R.
"Sexual dimorphism in malpighian tubules of Pteronarcys californica Newport (Plecoptera),"
Great Basin Naturalist: Vol. 35
, Article 14.
Available at: http://scholarsarchive.byu.edu/gbn/vol35/iss4/14