Comparative anatomy of the Mallophagan head.
- 1The construction of a generalized Mallophagan head, by which main types within the suborder may be interpreted, is preceded by a description of a generalized insect head and tentorium to introduce terminology and emphasize the importance of tentorial pits as “landmarks” on the cranium.
- 2i. A description is given of the internal skeleton or tentorium of Psocoidea. Nine distinct modifications (including total absence) are found to exist in species representing 36 Mallophagan genera and typical genera of Psocoptera and Anoplura. The species examined are listed (pp. 351 & 352). ii. A tentorium is present in all Mallophaga; it is complete (apart from dorsal arms) and variably developed in Amblycera; incomplete, though comparatively uniform, in Ischnocera. A bridge is present in Amblycera and Ischnocera, but only posterior pits remain in Rhyncophthirina. There is no trace of a tentorium in Anoplura. Variations of tentorial form consist mainly of reduction and loss of sclerotization. iii. Anterior tentorial pits in Psocoptera and Mallophaga arise in the pleurostomal part of the sub-genal suture (primitive position, common in lower Pterygota), not the clypeo-frontal suture (less primitive, characteristic of the majority of Pterygota). iv. The ventral position of the anterior tentorial pits in Mallophaga is attributed mainly to dorso-ventral flattening of the head. v. A ligament attached to the anterior tentorial arm and innervated by theclypeal nerve has been found in Mallophaga and is called the clypeal ligament; its origin on the dorsum of the head appears to represent a point on the posterior limit of the clypeus; in Ischnocera it serves to attach the anterior arms, which are disconnected from the rest of the tentorium, to the dorsum of the head. Reasons are given to show that these anterior arms are not true dorsal tentorial arms. A muscle, homologized with the ligament of Denis (1928) present in apterygote Collembola, is described, in Amblyceran Psocoidea only. vi. Tentorial variations and distribution of muscles and ligaments of the Psocoidean head are summarized (p. 402).
- 3An account of the obturaculum (sensu Stojanovich, 1945) of Anoplura, on which muscles of the trophic stylets originate, is given, since its appearance may relate to loss of the tentorium in these insects. Further, it may be homologous with fibrous tissue surrounding the head ganglia of Ischnocera and Rhyncophthirina which is unusually well developed compared with Psocoptera, Amblycera (and other insects) and may therefore have phylogenetic value.
- 4A lacinial gland is described; it is structurally similar and equally well developed in Amblycera and Ischnocera in spite of variations in other trophic structures, but absent in other Psocoidea.
- 5i. Examination of various types of Mallophagan head yielded data which, together with those of the positions of tentorial pits and nature of the sclerotized bars or sutures, have enabled a generalized Amblyceran and Ischnoceran form to be built up by progressive steps from a primitive hypognathous insect head. This has suggested that the sutures and areas of the Mallophagan head can be homologized with those of a typical insect head. ii. The Ischnoceran form differs from the Amblyceran only in modifications relating to the position of the posterior tentorial pits, extension of the anterior region of the head and enlargement of the clypeo-labral suture to form a pad of tissue (the pulvinus of Cope, 1940). iii. The majority of Amblyceran heads depart little from the generalized form. iv. Modification of the pulvinus and division of the clypeus (Philopteridae) to form a chitinous plate (“clypeal signature”, homologized with an ante-clypeus) contribute to the differences between the Goniodid, Trichodectid and Philopterid Ischnoceran heads which are all otherwise fundamentally similar to the generalized form. v. Comparisons are made, chiefly by means of diagrams (figs. 56–60) of other interpretations of the Mallophagan head; Harrisoniella by Cope (1940), and Trichodectes and Goniodes by Kéler (1938, 1939).
- 6i. The phylogenetic significance of tentorial form is discussed, bearing in mind the inadvisability of putting too much importance on deductions based on comparisons of one morphological character. Though structural variations are apparently caused by reduction, migration, or loss of muscles associated with the tentorium, they do not appear to be correlated with modifications of mouth-parts induced by different feeding habits. Distinct types of tentorial form exhibit progressive reduction throughout the Psocoidea. The tentorium is almost identical in lice known (on other morphological characteristics) to be closely related but becomes markedly different in genera of more remote ancestry. No parallel sequence of tentorial modifications was detected within the groups of lice examined. An attempt is made to use tentorial form as a guide to the possible derivation of Psocoidean orders (summarized in Table I), assuming that reduction and loss have not taken place independently in Rhyncophthirina and Anoplura. ii. The position of the tentorial pits and retention of the muscle in Amblycera equivalent to the tensory ligament of Denis (1938) in Collembola is characteristic of primitive Pterygota, such as Psocoptera, and thus suggests that Mallophaga are survivors from a very primitive insect group. iii. Degree of development of the tentorium indicates that the evolutionary sequence of members of the Psocoidea may be: order Psocoptera, and then, of the order Phthiraptera, the superfamilies Amblycera, Ischnocera (with Trichodectidae more primitive than Philopteridae), Rhyncophthirina and, least primitive of all, Anoplura. iv. Comparisons of arrangement and occurrence of muscles in the head of Mallophaga and Anoplura indicate a close relationship between Anoplura and Rhyncophthirina, and that they are probably descendants of an ancestor common also to Trichodectidae (rather than Philopteridae) of the Ischnocera. Muscle arrangements in the head of Trichodectidae are thought to be more primitive than those in Philopteridae. v. The more than usually well-developed fibrous tissue around the head ganglia of Ischnocera and Rhyncophthirina, and its still further development (obturaculum) in Anoplura, supports the view that these groups share a common origin. If loss of the lacinial gland, which is described in all other Mallophaga, did not take place independently in Anoplura and Rhyncophthirina, then the two latter groups may be more closely related to each other than either is to Ischnocera. Retention of posterior tentorial pits in Rhyncophthirina indicates that the surviving genus of this group is possibly more primitive than extant Anoplura in which no trace of a tentorium remains. vi. Since tentorial structure in Ischnocera is uniform in all species examined, and distinct from Amblycera, in which many variations occur, all members of the former superfamily are themselves more closely related than individuals within the other superfamily are to one another. vii. The foregoing conclusions are thought to be, in the main, consistent with extant views of Mallophagan phyletics. It is believed, however, that the following suggestions arising out of the present work on the morphology of the head have not been put forward before, (a) Ischnoceran ancestors may have been derived from some Boopid-like form, (b), the Boopidae and related families (Gyropidae and Trimenoponidae) are less primitive than most of the bird Amblycera. Further evidence has appeared which seems to link the Rhyncophthirina closer to Anoplura than Ischnocera in a position between these two groups, if it be assumed that the characters taken into account in these assumptions were not developed along independent lines.