The Hog Louse <i>Haematopinus suis</i> (Phtiraptera) Midgut Morphology and Function. Comparison With Hemiptera and Holometabola

The taxon Paraneoptera, comprising Condylognatha (Hemiptera e Thysanoptera) and Psocodea (Phthiraptera and Psocoptera), is paraphyletic, according to recent genomic studies. The midgut morphology and digestive physiology of Haematopinus suis (Phtiraptera) are described here and compared with similar data from Hemiptera and Holometabola. H. suis (Phtiraptera) midgut cells, in contrast to Hemiptera, lack microvillar-associated membranes and have a digestive enzyme secretory mechanism of the type microapocrine, like many Holometabola. Also, like Holometabola, and different from Hemiptera, the main endopeptidases of H. suis are serine endopeptidases (trypsin and chymotrypsin), instead of cysteine endopeptidases. The major difference of H. suis in relation to Holometabola is the absence of a peritrophic membrane (PM), thought to be a consequence of the fact of PM cost of production in this minute insect out weights the benefit of midgut compartmentalization. Our data favor the view that Phthiraptera is closer to Holometabola than to Hemiptera, agreeing with the alleged paraphilia of Paraneoptera.