W. T. M. Forbes.
On the Tympanum of Certain lepidoptera.
Psyche 23(6):183-192, 1916.

This article at Hindawi Publishing: https://doi.org/10.1155/1916/16801
CEC's scan of this article: http://psyche.entclub.org/pdf/23/23-183.pdf, 1412K
This landing page: http://psyche.entclub.org/23/23-183.html

The following unprocessed text is extracted automatically from the PDF file, and is likely to be both incomplete and full of errors. Please consult the PDF file for the complete article.

19161 Fmbes~On the Tympanum of Certain Lepidoptera 183 of the worker is longer and has a reflected occipital margin which is lacking in pubiventris and anguata.
The observations of Mr. Gaste and Dr. von Ihering throw light on one of the methods of ant dispersal. Of course, the estab- lishment of a species of ant on an island does not require the conveyance of a whole colony as in this instance, since a single fecundated female enclosed in a vegetable cavity might, if trans- ported in a similar manner, suffice for the establishment of a species.
ON THE TYMPANUM OF CERTAIN LEPIDOPTERA.
BY WILLIAM T. M. FORBES,
Ithaca, N. Y.
At the base of the abdomen in many Lepidoptera there is a -curious and complex organ, which has been referred to but rarely in the literature, and which is known, in the Geometridse, at least, .as the tympanum. So far as I have discovered, its function is unknown, but its location and general
structure justify the guess that it is
the resonator for an auditory organ,
corresponding to the well-known one
in the Hemiptera.
It should prove an
interesting problem for the histologist
and The present article
will not consider its function, but will outline its structure in the principal
families of Macrolepidoptera, and offer
.a couple of suggestions as to their
relationship.
A brief examination shows that there
:are several analogous, but not homol-
(ogous organs of this type in the vari-
ous families, indicating parallel lines of descent from a form that presumably t,hf::; ~ ~ ~ ~ ~ m ~ t ~ ;
had neither, but may have possessed a
Apatelodes twrefacta (Euptero-
wholly internal sense-organ,-which tid~) showing primitive macro- lepidopterous arrangement of
served as a stimulus to their forma- scIerities.



================================================================================

184 Psyche [December
tion,-in connection with the first spiracle. The first type of tympanum is that more or less familiar in the Geometridse, which is used by Prout (Genera Insectorum, fasc. 103) as the most fundamental distinguishing character of that family. This ap- pears superficially as a simple hollow balloon located immediately below the spiracle, opening to the exterior in the fold that nor- mally exists in the sclerite of the abdomen facing the hind coxa. It is usually more or less ellipsoidal, and opens conspicuously on the side, forward and outward. In many cases the two tympana are so large as to be separated from each other only by membrane, and look like a pinhole clear through the moth,-as is very con- spicuous in Eudule and many Acidaliinse (Sterrhidse). In the Ennominse the organ is smaller and less conspicuous externally, but entirely of the same character, and it exists even in the lower. (Enochrominse (Monocteniidse) . In this type the spiracle lies on the upper edge of the organ, and the dorsal part of the segment is normal.
Internally (Fig. 6) the structure is seen to be somewhat more complex. The tympanum itself may be compared roughly to a kettle-drum with edges curving inward strongly to meet the edge of the membranous area that represents the head, and faces for- ward. (It has been turned back in the figure to show the struc- tures between it and the thorax.) Running almost across this head there is a tapering chitinous tongue, lying in the membrane, to the tip of w-hich are attached a couple of muscles, the soft fleshy connection with the nerve (B), which histological study, is likely to prove the end-organ itself, and a variety of air-sacs and fine tracheae. In fact the whole organ is enveloped in a system of air- sacs, which have mostly been removed in the specimen figured. The external opening of the apparatus is on the side that lies against the body-wall,-as if a large hole had been cut in the side of the kettle. The nerve-supply appears to be from the anterior seg- mental nerve of the first segment of the abdomen, which runs back from the ganglion, which is fused with the metathoracic one. This nerve runs past the tympanum anteriorly, and then over the spir- acle and up to supply the dorsal musculature, etc. The part of it between the tympanum and spiracle is surrounded by large air- sacs, which connect directly with the spiracle. The European genus Cimelia is generally placed in the Geomet--



================================================================================

Forbes~On the Tympanum of Certain Lepidoptera Fig. 6.
Tympanum with its principal muscles and nerve connections, of the Geometridse (drawn from Biston ursaria and Phigalia titea). Only two of the
numerous air-sacs are indicated, the one lying ventrad of the nerve, and along which the nerve runs from the tympanum to the spiracle, and the large double one connecting the tympana. The latter appears ventral in the dissection, which was opened out on the middorsal line, but in fact lies above the digestive system. When the dissection is first opened it is normally filled with air and may be the most conspicuous object in the field.
ridse, but appears to be omitted in Prout's consideration of the (Enochrominse (Genera Insectorum, fasc. 104). A superficial study of the single dry specimen available shows that there is no true tympanum, either of Geometrid or any other type, but there is a slight thickened bar that may represent the bar across the drum-head of the tympanum, lying in the membrane of the ven- tral surface. There is no definite suggestion of Notodontid struc- ture; and I believe that the genus represents a new family type, leading up to the Geometridse from something intermediate be- tween the Dioptidse and Notodontidse,-probably direct from the unknown Geometrid-Bombycid ancestral form. There seems to be nothing in venation or mouth-parts to contradict this refer- ence.




================================================================================

186 Psyche [December
For the normal, and primitive condition in the Lepidoptera (except the very lowest Tineoids and Jugatse), we may take Apa- telodes, a somewhat isolated genus of the Bombycid stem (Fig. 1). Here the metathorax is moderately developed, and there are no specializations which do not occur all the way from the Cossidse to the Saturniidse. The scutum and scutellum are normal, the cord connecting the scutellum with the inner margin of the wing is somewhat wavy, but forms no distinct lobe, the epimeron is V-shaped as in the mesothorax, and though too lightly chitinized to be quite sure of its boundaries shows no sculpturing except a slight articular surface where it meets the tergopleural line of the abdomen. The postscutellum is not sharply set off from it, and the entire structure is essentially as in the mesothorax. The first segment of the abdomen is, as usual in Lepidoptera and related insects, largely membranous; the tergum is a rectangle of mem- brane, enclosed in an anterior band which articulates subdorsally with the thorax, a posterior band, extended well down on the sides, but not reaching the ventral region, and a pair of heavily chitinized lateral grooves;-the latter seem to represent the ter- gopleural suture, so that the narrow edge of chitin below them should be considered the pleurite. The front end of this groove runs to a shallow pocket between thorax and abdomen. The lateral region is wholly membrane, containing a normal spiracle. Ventrally the first two segments are not distinguished, and are Fig. 2.
Same view of Hemer-
ocampa leucostiqma (Liparidse),
showing second type of tym-
panum.
front where they lie against, and are
slightly grooved by, the posterior
coxse. The external modification to
form the first or Geometrid tym-
panum is merely the enlargement
of the coxal groove into an ample
cavity, and the distortion of other
structures to make room for it.
The second type of tympanum oc-
curs in the Arctiidse, Lymantriidse,
Syntomidse, Pericopidse, etc. (Fig. 2 .) In this the thorax is undisturbed,
though in the forms I have studied
there is the usual lack of definiteness
of the epimeron; the abdomen dor-




================================================================================

sally and ventrdy is as before, but in the pleural region above the spiracle there has developed a bulls, exactly like that of the Geomet- ..
rid= externally, but instantly distinguished from it by the fact the spiracle lies on its lower edge. A curaory examination suggests that it has developed out of the remaining rudiments of pleurites. The thorax shows more or less trace of the curious modification described for the Noctuidte, but the venter of the abdomen is- typical.
Fig. 3. Same view of Catocala arta, showing third, or wual Noctuid type of tympanum. The cover, or aiu~a of the hind wing is drawn forward, and the posterior-edge is turned back to expose the spiracle and structures within the tym- panmn.




================================================================================

188 Psyche [December
The third, or Noctuid type, is perhaps the most interesting of all, being at once the most complex, and that occurring in the largest family. (Fig. 3.) Here there is no simple rounded bulla, either subdorsal or subventral, but the whole spiracular region is deeply excavated, the excavation including the posterior region of the thorax as well. This cavity in the more typical case is roofed over by the alula of the hind wing, which is largely developed, especially in such Quadrifidze as Catocala and the Euteliin~, and is enclosed behind by a large flap, developed behind the spiracle. It seems to be the same pleurite that has formed this and the bull% of the Arctioids, but the development has been behind, not above the spiracle, and is of a wholly different character, as the figure shows. Another interesting development is a deep fovea in the parapleural region of the thorax, across which the suture between postscutel- lum and epimeron may sometimes be traced. In this series the epimeron, often heavily chitinized, tends strongly to break up, and to develop more complex sculpturing, as is notable in Cato- cala. Alypia and the Notodontid~ (Fig. 5) show interesting variants of this type. In the latter the lateral cavity is present, but slightly developed, and the alula is rudimentary, not covering Fig. 5.
Same view of Datana
ministra.
it, the lateral flap behind the spira-
cle is also absent, though the material
from which it forms is indicated as
a chitinized area about the spiracle
that already tends to overgrow the
lateral groove. The parapleural fovea
is well marked, however, and already
undercuts a portion of the para-
pleura, which makes a stiff and solid
connection with the abdomen (partly
broken in the figure). This type
could well represent the point of
origin of the Noctuid one, but is
wholly unlike the simple structure of
Apatelodes. In Alypia (Fig. 4) we
have a second rather simple modifi-
cation. The parapleural fovea is well
marked, but the region about the
spiracle is reduced, without any spe-




================================================================================

19161 $orbeeon the Tgmpanum of Ce~tain Lepidopttm cia1 sclerites, and in this case the hol- low area which normally exists dor-
sally between thorax and abdomen is
very much enlarged, extending back
into the second segment, as a thin-
walled balloon. In all variants of
type three this fold seems much bet-
ter developed than in the primitive
condition.
As to the evolution of these forms I
have little doubt that I am correct as
to the primitive condition, since it
occurs generally in both Micro- and
Macrolepidoptera; from it I should
derive type three (in its simpler No-
todontid subtype) directly, and from
that the more specialized subtypes
and type two. Type one seems, how-
ever, independent. Working this out
as a genealogy we will have a first
dichotomy between the primitive con-
Fig. 4. Same view of Alypia
octomacu,lata (Agaristidze) . The
internal expansion of the fold
between thorax and abdomen
is indicated by the dotted lines.
A strong suggestion of this
structure occurs in some Noc-
tuid~.
dition (plus type one) and types two plus three. This corresponds
(so far as I recall, exactly) with the division between upright and flat egg, and between the caterpillars with hooks in a single length and those alternately of two lengths, and in those two particulars would dispose of the anomalous position of the Notodontidze (ex- cluding Apatelodes) . It would leave, however, the Notodontid~ as a solitary trifid family in a mass of Quadrifid~, and would simi- larly maroon the Lasiocampidze in a wilderness of trifid~. Possibly the importance of the position of M2 in the fore wing is less than it has been considered. Already we give little weight to it in the hind wing, and in certain Hydriomenid Geometridze, such as Eudule.
FAMILIES EXAMI~ED.
Primitive type.
Microlepidoptera. Bomb ycid~.
Cossidze. saturniid~.
Eucleidze, etc.
Sphingid~.
Eupterotid~ (Apatelodes) . Lasiocampidze.



================================================================================

190 Psyche [December
Type I ( subventral tympanum). Pericopidz. Geometridx only. Herminiini and Epizeuxis of the Noctuidz.
Type 2 (subdorsal bulla) .
Lymantriidz. Type 3 (parapleural fovea,) Arctiidz. Notodontidz.
Nolidz Noctuidz.
Syntomidz. Agaristidz.
It should be noted that Brues and Melander, in their key to the families of Lepidoptera erroneously refer to this organ in the Pericopidz and Geometridx, as the spiracle; it is perfectly distinct from the spiracle, lying above it in the former, and below in the latter case. So far as I know the spiracle itself is always small and normal, and varies very little in position. It should be noted, however, that the "tympanic opening " of the Dioptide (PSYCHE, XXI, 59, alternative 32) is not homologous with thatof the Geomet- ridx, but is the rudimentary structure noted in Apatelodes, and which becomes enlarged in the Agaristid~. In the case of the Pericopide (alternative 48) the separation from the Arctiidz is slender, while the majority of Noctuidz of course completely lack the bulla. The key would really be cleared up by transferring Haploa to the Pericopidz, where it would not be as wholly out of place as one might think. The Pericopide certainly belong here rather than in the neighborhood of the Notodontidx or Bomby- tide.
A study of the base of the abdomen in certain Lepidoptera sug- gests :
1. The Notodontidz are better placed near the Noctuidz than near the Geometridz and Bombycidz.
2. Apatelodes is not Notodontid, but may be Eupterotid, agree- ing in general with the Bombycid series. 3. The Lymantriidz, Arctiidz, Nolide, Pericopidz and Syn- tomidz are undoubted
close relatives, agreeing in a unique
specialization.
4. The Noctuidz, Agaristidz and Notodontidz are similarly



================================================================================

19161 Forbeeon the Tgmpanum oj' Certain Lepidoptera 191 grouped, and no reason has developed to separate them widely from the preceding group.
5. The Bombycid~, Lasiocampidze, Saturniidze, Sphingid~, Apatelodes, and their relatives also form a distinct group, but as they are associated by the absence of specializations in the struc- tures studied, they may be justly divided up on other characters. 6. The tympafiic bulla of the Geometridze is not homologous with that of the Arctiid=, etc., being developed on the opposite side of the spiracle.
7. A new subfamily, Cimeliinm, is needed for the genus Cimelia, based mainly on the following characters: Tympanum absent; Sc of hind wing not angulate at base, without a brace con~ecting it with the base of the frenulum, closely parallel to R to a point well beyond the end of the cell.
Structure otherwise as in the lowest
Geometridze (CEnochromin~) .
SUPPLEMENTARY NOTE: Since writing this article, the Diop- tid~, Thyatiridze, and Drepanidze have been examined. The Dioptide show no tympanum of any kind and will make a first exception to the rule that uniordinal hooks on the prolegs are co- ordinated with a tympanum of Arctiid or Noctuid type. The egg is described as spherical, and so may be of either type. In the Thyatirid~ and Drepanid~ there is a new type of tympanum; the tergopleural suture is modified into a large cavity, opening backwa~d, which lies beneath the spiracle, and there is a large double chitinized structure subventrally which does not open to the exterior unless it communicates with this tergopleural opening. The structure is so complex, and so similar in the two families as to make their association almost certain, especially as the larval and venational characters are rather similar. A = Lower edge of subdorsal muscle with nerve-supply. A1, A2 = First and second segments of abdomen. A1 ant. n. =Anterior nerve trunk of first segment of abdomen. B = Supposed sensory end organ of tympanum. conn. = Connective.
ex. = Coxa.
epm. = Epimeron (of metathorax) .
eps, = Episternum (of metathorax) .




================================================================================

192 P8gche [December-
int. = Portion of digestive system.
m. = Meron of hind leg.
post. n. = Posterior nerve trunk of first segment of abdomen. ppl., psc. = Parapleura, or exposed lateral end of postscutellum.. sc" ., sc"'. = Scuta of meso-, and metathorax. sclf'., sclf". = Scutella of meso-, and metathorax. spl., sp2. = First and second abdominal spiracles. subal. = Subalar sclerite of metathorax. tymp. = Tympanum.
'
1. Side view of metathorax and base of abdomen of Aputelodes torre~uctu (Eupterotide) , showing primitive macrolepidopterous arrangement of sclerities.
2. Same view of Hemerocampa leucostigmu (Liparide), showing second type of tympanum.
3. Same view of Cutocula purtu, showing third, or usual Noctuid type of tympanum. The cover, or alula of the hind wing is drawn forward, and the posterior edge is turned back to expose the spir- acle and structures within the tynipanum. 4. Same view of Alypia octomaculuta (Agaristide). The internal expansion of the fold between thorax and abdomen is indicated by the dotted lines. A strong suggestion of this structure occurs in some Noctuid~.
5. Same view of Datanu ministra.
6. Tympanum with its principal muscles and nerve connections9. of the Geometrid~ (drawn from Biston ursuriu and Phiguliu titea). Only two of the numerous air-sacs are indicated, the one lying ventrad of the nerve, and along which the nerve runs from the tym- panum to the spiracle, and the large double one connecting the tympana. The latter appears ventral in the dissection, which was opened out on the middorsal line, but in fact lies above the diges- tive system. When the dissection is first opened it is normally filled with air and may be the most conspicuous object in the field.



================================================================================