The First Abdominal Segment of Embryo Insects. 123 
are to be regarded as modified cephalothoracic appendages and not rather 
as outgrowths of the pleural wall out of line with the legs. The isolated, 
fact, though interesting, can be of little service, till reinforced by more ob¬ 
servations. Of course no homology can be maintained between these wing- 
lik'e organs and the pleuropodia of insect embryos. 
In spider embryos the knob-sliaped appendages on the four basal ab¬ 
dominal segments are very distinctly defined. They have been described 
by Balfour [‘80], Locy [’86], Bruce [’87] and Morin [’87]. There can be no 
doubt, from the manner in which they arise, that these knob shaped ap¬ 
pendages are the serial homologues of the cephalothoracic appendages, but 
so vague is our knowledge of the segmental homologies between the 
Arachnida and Hexapoda, that it is impossible to say which of the four 
pairs of appendages corresponds to the pleuropodia. 
Bruce [’87], maintains that in spiders “ probably two abdominal append¬ 
ages are invaginated t.o form each lung-book.” According to Morin [’87] the 
first pair of knob-sliaped appendages become the covers of the lungs which in- 
vaginate at their bases; the second pair disappear completely, while the third 
and fourth pairs form the spinnerets, the ectoderm on the summit of each 
of the four protuberances invaginating to form the spinning glands. Bar¬ 
ring the question of homology, the observations on the pleuropodia of 
Cicada and Zaitha, given in the first part of this paper, make it easy to see 
how an appendage might invaginate to form a lung-book, as maintained 
by Bruce. 
As soon as we reach the Sympliyla and Thysanura, we no longer en¬ 
counter any difficulty in settling on the true homologue of the first abdomi¬ 
nal segment of insects. 
The remarkable synthetic form, Scolopendrella, has twelve pairs of legs. 
Ryder [’81], Packard [’81] and Haase [’87], agree that the first three pairs 
are to be regarded as the homologues of the three pairs of legs of the in- 
secta and that the three segments to which they belong, are to be regarded 
as the homologues of the pro-, meso-and metathoracic segments respectively. 
If this view be correct, and there is certainly nothing to militate against it, 
then we may regard the fourth postcephalic segment of Scolopendrella 
as the homologue of the first abdominal segment of insects. This being the 
case, the two ambulatory legs attached to this segment in the Symphyla 
are the homologues of the pleuropodia of embryo Hexapods. 
From the Symphyla, we pass to the Thysanura , a group comprising sev¬ 
eral forms of interest in connection with the pleuropodia of embryo insects. 
Appendages are known to occur on the basal abdominal segment in 
Campodea , Machilis and allied species, and in the Collembola. Through¬ 
out the Thysanuran group the basal abdominal segment is doubtless to be 
regarded as truly homologous with the first abdominal segment of the 
higher, or winged Hexapod r (Pterygogenea). Hence, a pair of appendages 
arising on this segment in the Thysanura as ectodermic evaginations, with 
mesodermic cores, in line with and at approximately the same time as the 
