• Head retracted within thorax
• Longitudinal incisions usually on dorsal side
• Head may be only several slender rods
• Respiratory system without spiracles or spiracles only on posterior abdominal segment
• Posterior spiracles usually bordered by 1 to 3 pairs of short lobes fringed with hair

Merritt, R W., K W. Cummins, and M B. Berg. An Introduction to the Aquatic Insects of North America. Dubuque, Iowa: Kendall/Hunt Company, 2008