What are abdominal appendages in apterygotes?

In apterygotes, abdominal appendages are considered modified remnants of ancestral walking limbs. Unlike pterygotes (winged insects), which lack pregenital abdominal appendages in their adult stage, apterygotes are characterized by the presence of one or more pairs of these structures.123

The specific types and functions of these appendages vary across groups:

• Cerci: Many apterygotes possess cerci, which are sensory appendages located on the 11th abdominal segment. These can be long and multi-segmented, short with a central duct, or modified into pincer-like structures for capturing prey.1
• Ventral Styli: Found in groups such as the Zygentoma and Archaeognatha (as well as the extinct Monura), these are modified limbs that serve as taxonomic markers.1
• Vesicles: One or two pairs of vesicles are present on certain abdominal segments in various apterygote groups, including proturans.1
• Specialized Organs: Some groups have evolved unique structures from ancestral limbs, such as the collembolan (springtail) furcula (a springing organ), a median tenaculum, and a tube containing vesicles.1

These appendages are essential for identifying and classifying different groups of primitive hexapods.13