Ultrastructural organization of the larval spiracles in Libellula depressa L. (Anisoptera: Libellulidae)

In the last larval instar (F-0) of L. depressa, 2 paired spiracles, in the form of elongated eye-shaped structures, are located in the anterior region of the mesothorax segment. A fine structural analysis of these spiracles under the scanning and electron microscopes reveals that each spiracle consists of a well-developed cuticular peritreme with a dorsal-anterior lip bearing a thin laminar coat and a ventral-posterior lip bearing a filter apparatus. The filter apparatus derives from a series of folds forming discrete groups adhering to one another to delimit empty spaces and producing a honeycomb-like structure. This structure is coherent with the need to avoid entry of water when the larva is submerged. The function of these spiracles during the insect development is discussed, noting that in anisopteran larvae the rectal epithelium, forming the so called branchial basket, is the main respiratory organ.