Fine structure of the midventral abdominal tracheal glands in banded woolly bear caterpillars (Arctiidae: Lepidoptera)

Caterpillars of Pyrrharctia isabella have four well developed tracheal glands located at the midventral anastomosis of the transverse tracheae in the third to sixth abdominal segments. The glands are approximately spherical, about 0.7 mm in diameter, and consist of a single, lobulated layer of very large glandular cells enclosing a sacculated reservoir. The haemal margin of the gland cells is characterized by extensive plasma membrane infoldings and the lumenal margin abutting the reservoir is deeply invaginated to form large microvilli-lined secretory cavities. The secretory cavities contain a lipoidal secretion that occurs in the form of fragmented plate micelles with a lamellar periodicity of about 55 Å. The secretory cavities are confluent with subcuticular spaces, from which the secretion can cross the lumenal cuticle into the reservoir and thence gain access to the exterior via the tracheal system. The most significant cytoplasmic features of the gland cells are an abundance of smooth endoplasmic reticulum and numerous small golgi complexes. Functional aspects of the cellular organization are discussed in relation to the possible role of the tracheal glands.