While out walking along the edge of a field here in southeast Michigan, I came across a handful of night-flowering silene (Silene noctiflora, Caryophyllaceae). While taking some photos, I noticed there were a number of bubble clusters on the plants:
These bubbles are produced by the nymphs of spittle bugs (family Cercopidae). As hemimetabolous insects, spittle bugs don’t have an egg-larva-pupa-adult cycle like holometabolous insects such as butterflies and bees. Instead, the eggs hatch into nymphs, which vaguely resemble adults. After each molt, the nymphs become more and more adult-like until they finally molt into adults.
The cuticles (exoskeletons) of hemimetabolous insect nymphs tend to be poorly sclerotized (hardened). The insects are relatively soft and squishy, and lack the harder, better-sclerotized cuticles of adults. Spittle bug nymphs are also much less nimble than adults. Because of the weaknesses of these young bugs, they could be easy prey for predators. As a defensive mechanism, the nymphs produce these bubble clusters. The bubbles give the nymphs a moist environment to hide in as they suck on plant sap. Once they molt into sturdier, more agile adults, they no longer produce spittle.
Despite the name “spittle” bug, these nymphs don’t produce their spittle from their mouths. The spittle is actually secreted from their anuses (Triplehorn and Johnson 2005). These bugs are also called toadhoppers because of the vague resemblance the adults have to toads.
Triplehorn, C.A. and N.F. Johnson. 2005. Borror and DeLong’s Introduction to the Study of Insects. Seventh Edition. Thomson Brooks/Cole, Belmont, CA.