M. F. Crowett.
The Tracheal System of the Mature Larva of Pyrausta nubilalis Hubn.
Psyche 36:332-357, 1929.

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Psyche
[December
THE TRACHEAL SYSTEM OF THE MATURE LARVA
OF PYRA USTA NUBILALIS HUBN.1
The tracheal system of the larva of the European corn- borer, Pyrausta nubilalis Hubn. (Lepidoptera ; Pyralididse) is of the peripneustic type. There is one pair of functional spiracles on the thorax, and eight pairs on the abdomen. (Fig. 1). These spiracles open into short stout tracheae which lead, on each side, to a large longitudinal trachea which runs throughout the length of the body, and from which branches go to the various body organs. The Spiracles
The spiracles are of the type described by Kranchner, according to Packard (1909), as "bearing lips." The lips are represented by a chitinous ring surrounding the ellip- tical opening through the body wall. They are "roof-like," or bent inwards, and are hairy. (Fig. 2). From the spir- acle a short stout trachea joins the adjacent longitudinal trunk. (Fig. 3).
The Tracheal Closing Apparatus
The tracheal closing apparatus (Fig. 3) surrounds the short trachea between the spiracle and the main trunk. On one side of the trachea, extending about half around it, is a chitinous band, the closing bow. Attached to one
end of this bow is one extremity of the dosing lever. The closing lever runs parallel with a line joining the ends of the closing bow, to a point almost halfway between the ends of the bow. Here the lever turns abruptly away from the



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19291 Trachea! System of Pyrausta nubzlalis . 333 center of the trachea at approximately a 15ght angle. The projecting arm thus formed is somewhat larger than the other arm and swells slightly toward its free end. The lever is chitinous. Between the apex of the lever and the end of the closing bow opposite to that to which the lever is attached is a third chitinous piece. This is the closing band.
The contraction of a muscle extending between the free arm of the lever and the closing band causes the lever to move. The attachment of the lever to the closing bow be- comes the fulcrum, and the action of the muscle pulls the apex of the angle of the lever inward, causing the space between the lever and the closing bow to become constricted. The closing ba,nd is so attached to the apex of the angle that the end of the band is depressed inward, causing the space between it and the closing bow to be constricted. The trachea is thus pinched together.
This structure is cast with the exuvia when molting takes place.
Kranchner (1880), who first described the tracheal clos- ing apparatus of insects, expresses the opinion that it originates as a local thickening of the tsenidium. The description of the spiracles and closing apparatus is taken from a former paper, (Crowell, 1926). The Longitudinal Tra,chese
In the embryo the tracheal system arises as ectodermal invaginations which grow deeper, and from which branches are sent out. One running forward fuses with one run- ning backward from the next anterior spiracle, and one running backward fuses with one running forward from the next posterior spiracle. Thus, longitudinal connections are formed between the spiracles, and these connections form the main longitudinal tracheae. In the thorax there is a longitudinal system on each side dorsal to the main system. It is formed by the joining of dorsally ~nning branches.
The longitudinal tracheae lie on each side in an area be- tween the small dorso-recti muscles and the great ventro-



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334 Psyche [December
recti muscles. In this area there are no longitudinal muscles, but it is crossed by sterno-tergal muscles. Lying in front of all of the spiracles, except those in the prothorax, is a sterno-tergal muscle, and the longitudinal trachea except in the mesothorax and in front of the first abdominal spiracle, passes between these muscles and the body wall. In the prothorax the sterno-tergal muscle lies behind the spiracle. In the metathorax a large branch from the main trachea passes beneath this muscle. In the meso- thorax the muscle is present and a trachea arising in a manner similar to the branch in the metathorax passes between it and the body wall. Thus in each body segment through which the main trachea runs it is held to the body wall by a muscle passing over it. (Fig. 4. The trachea is much longer in proportion to its thickness than here in- dicated).
According to De Guyse (1926) the primitive position of the spiracle located on the prothorax was in the membrane between the prothorax and mesothorax, and it has moved ahead to its present position. That would account for the fact that the sterno-tergal muscle lies behind the spiracle rather than in front of it, as in the abdomen. DeGuyse (1926) explains the positions of the muscles in front of the first abdominal spiracle, and in the mesothorax, from a consideration of the larvae of Sthenopis (Hepialidse) , as follows :
The thoracic spiracles are situated at a much lower level than those of the abdomen.
In the abdomen the anterior
lateral branch arising from the short spiracular trachea passes under the sterno-tergal muscle and anastomoses with the posterior lateral branch from the next preceding seg- ment. The anterior lateral branch from the first abdominal spiracle passes as usual under the muscle, but since the second thoracic spiracle is at a much lower level, its pos- terior lateral branch fails to meet the anterior lateral branch from the abdomnial spiracle. The latter loses it- self among the dorsal muscles. At the same time the lower visceral branch is found greatly enlarged, and anastornosed with a posterior branch of the first thoracic spiracle. The



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19291 Tracheal System of Pymusta nubilalis 335 second pair of spiracles thus loses its connection with the general respiratory system and ceases to function. That means that the apparent branch in the metathorax, arising from the apparent main trachea and passing be- neath the muscle, is, in fact, the main trachea, and the apparent main trachea is a branch. But it is hard to explain the tracheal branchings in the mesothorax on this basis.
The branch from the first abdominal spiracle passing forward beneath the sterno-tergal muscle, does not, in Pyrausta nubilalis "lose itself among the dorsal muscles," but becomes a part of the dorsal longitudinal system, send- ing branches to the metathoracic leg and to the wing-bud. Figure 5 and figure 6 show the branches arising from the first and fourth abdominal spiracles, respectively. In the case of the branches arising from the first abdominal spiracle it is evident that the presence of the appendages of the metathorax, legs and wings, has made necessary more tracheal branches than are called for in the fourth segment where such appendages are absent.
But a study of the branches arising from the fourth abdominal spiracle fails to show any that takes over the role of the main trunk, if we consider that the branch from the first abdominal spiracle from which spring the tracheal supplies to the alimentary tract, the wing-bud, and leg, to be the anterior branch, or "main tracheal trunk" from that spiracle.
The apparent main tracheae, the "lower visceral branch" of de Guyse, does not meet the posterior branch from the prothoracic spiracle until after it throws out branches in the mesothorax that are at least analogous with the branches from the first abdominal spiracle. (Fig. 7). It is rather difficult to see how these branches arose as sec- ondary branches from the lower visceral branch from the first abdominal spiracle and the posterior branch from the prothoracic spiracle.
The Transverse Trachese
The longitudinal tracheae of each side are connected by transverse tracheas in every spiracle bearing segment, ex-



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336 Psyche [December
cept, possibly, the first abdominal. There is one in the mesothorax, although that segment does not bear a func- tional spiracle. There is one in the head. The transverse trachea in the head lies in front of the supraoesophageal ganglion and just behind the body of the tentorium. The posterior arms of the tentorium pass above the trachea, close to its junctions with the main longitudinal tracheae. This transverse trachea lies above the commis- sures between the supra- and suboesophageal ganglia. (Fig. 8).
Peterson (1912) describes and figures a cross trachea in the larva of the tomato-worm, Protoparce carolina, lying above the commisures between the supra- and sub-ceso- phageal ganglia, but in the tomato worm this trachea crosses the suboesophageal ganglion. In Pyrausta nubilalis it lies well in front of the ganglion. Peterson says nothing about the relation of this trachea to the tentorium, but from his figure, the trachea must lie well behind it. The next cross trachea, which I have numbered 2-a, is a secondary branch.
The primary branch from which it.
springs comes from the main trachea just after it leaves the prothoracic spiracle and its other branches will be de- scribed later.
Trachea 2-a runs on the outside of the muscle layer, and beneath the central nervous system.
In the prothorax there is a transverse system of tracheae dorsal to the alimentary tract. These tracheae are ar- ranged in such a manner that I term the system the diamond. (Fig. 9).
A large branch arises from the thoracic spiracle. It runs forward a short distance, then branches into two good sized tracheae, the larger being in front. Each of these branches meets a corresponding branch from the opposite side, and the double Y thus formed gives the shape that justifies my name for the arrangement. From the forward apex of the diamond branches go into the head. From the posterior apex a long branch goes backward and joins a branch from the main trunk that runs backward from the prothoracic spiracle Thus, counting each branch separately there are



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19291 Tracheal System of Pyrausta nubilalis 337 three transverse connections crossing the ailmentary tract between the prothoracic spiracles.
Peterson (I912), in the tomato-worm, finds that in all cases except the transverse tracheze described above, the cross trache~ lie ventral to the nervous system. This is not always true in Pyrausta nubilalis.
Just behind the prothoracic spiracle there is a trachea, which I have numbered I. This lies on the inner surface of the muscle layer. It crosses the nerve cord dorsally, ter- minating, usual~y, in the prothoracic ganglion. Near the base of the ganglion the trachea meets the corresponding trachea from the other side, and a cross connection, dorsal to the nervous system, and ventral to the alimentary tract, is formed (Fig. 10). Trachea 1 varies. It may be much reduced, forming only a very weak connection with the central ner~~ous system, with the corresponding trachea of the opposite side entirely absent (Fig. 11). Sometimes the trachea to the ganglion is obviously from the left side, with the trachea from the right meeting it (Fig. 121, and sometimes the opposite is true. The branches from trachea I usually supply the muscles, but in one specimen a large branch ran to the nerve cord in front of the mesothoracic ganglion. (Fig. 13). In the figures the size of the tracheal connections with the ganglion is exaggerated. Trachea 1 lies, apparently, at the junctions of the longi- tudinal muscles of the prothorax with those of the meso- thorax, and is a useful landmark in locating the limits of these segments.
The next transverse trachea I have numbered 2 (Fig. 14). It lies in the mesothorax on the outer side of the muscle layer. It passes beneath the central nervous system just in front of the mesothoracic ganglion. In relation to the muscles and the nervous system and to trache~ I and 3, which will be described next, it is, if not h~mologous to trachea 2-a, analogous to it.
Trachea 3 lies on the inner surface of the muscle layer and approximately marks the limits of the mesothorax be- hind and of the metathorax in front. It is a transverse trachea much as is trachea I. A branch to the mesothoracic ganglion meets a branch from the opposite side, forming a



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338 Psyche [December
cross tracheal connection dorsal to the nervous system and ventral to the alimentary tract (Fig. 15). Trachea 4 lies in the metathorax. It is on the outside surface of the muscle layer and crosses just in front of the metathoracic ganglion. It is similar to trachea 2. The system of numbering is now evident. Beginning with the first trachea behind the prothoracic spiracle lying on the inside of the muscle layer, extending toward the mid- ventral line of the body, which trachea I have called I, the trachez in each segment behind this that lie on the inner surface of the muscles at about the line between the seg- ments have uneven numbers. Those transverse trachez lying in the segment and on the outer surface of the muscle layer have even numbers.
Trachea 5 is not a transverse trachea; and the trachez in the abdomen analogous to trachez I, 3 and 5, are not transverse trachez.
Trachea 6 lies in the first abdominal segment. From its position on the outside of the muscles and from the dis- tribution of its branches, which will be discussed later, it is analogous, if not homologous to the other trachez of like position (those with even numbers). But unlike trache~ 2 and 4, and unlike the corresponding trachez in the ab- dominal segments, trachea 6 is not, as a rule, a transverse trachea.
In one specimen I found a connection with the corresponding trachea from the opposite side. I doubt if
that was typical. But it must be considered in connection with the transverse trachez because of its position. (Figs. 16, 17).
Behind the first abdominal segment there is a transverse tracheal connection in each spiracle bearing segment. From the second to the seventh segments incl~~sive, this trans- verse trachea is analogous to trachez 2 and 4, except that they lie behind, rather in front of, the ganglia. In the eighth segment there is a stout connection between the spiracles lying above the alimentary tract. Peterson (1912) in speaking of the tomato worm says: "In the case of the eighth abdominal segment, only one minute dorsal cross trachea was found, while in the pro- thoracic region two distinct cross trachez were observed.''



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19291 Tracheal System of Pyrausta nubilalis 339 Pyrausta nubihlis differs from the tomato worm in that the cross trachea in the eighth segment of the abdomen is not minute, but is a stout connection. In the prothorax the connections are apparently similar? but Peterson shows no tracheal connection between the posterior apex of the diamond and the main trunk.
Trachez from the Prothoracic Spiracle
The arrangement of the trachez from the prothoracic spiracle (Fig. 181, from which the organs of the prothorax and head are supplied, is much more complicated than that in the regions of the body behind the prothorax. It is easier to describe these trachez as from the prothoracic spiracle than to attempt to correlate them with the study of the tracheal supplies to various systems of organs. From the spiracle two large branches extend towards the head.
One of these I believe? for reasons to be set forth later? represents the main longitudinal trachea? or the an- terior branch from the spiracle. The other branch forks a short distance in front of the spiracle? and its inner branch forms the stem of the Y which makes up one side of the dorsal transverse trache~ to which I have referred as the diamond. The outer branch goes forward into the head. There it branches? and apparently supplies the great mandi- bular muscles lying in the side of the head. One branch was traced to the region of the base of the antenna, but I was unable to follow it into that organ. The other large branch going into the head from the spiracle branches almost immediately. This first branch sends out a small branch, and three larger branches of about equal size. The small branch goes toward the head. The branch nearest the head of the other three branches passes at once to the outer surface of the muscle layer. It runs toward the middle of the body, and sends a branch to the prothoracic leg. It continues, and sends a branch up- ward to the prothoracic ganglion. Then it meets the cor- responding trachea from the opposite side? forming a trans- verse connection ventral to the central nervous system. In its position and its branching to the ganglion? but not to



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340 Psyche [December
the leg, it resembles the transverse trache~ to which I gave even numbers, and is the trachea which I designated as 2-a. The middle of the three branches runs on the inner sur- face of the muscle layer and sends branches to the pro- thoracic and subesophageal ganglia, and to the muscles. The hindmost branch also runs on the inner surface of the muscle layer. It crosses the muscles, and bending ven- trally sends a branch to the prothoracic leg. The middle and hindmost branch, taken together, are, in their position and distribution so similar to trachea 3 that I believe them to be at least analogous to the trache~ with uneven numbers. Therefore I have numbered them together as branches of trachea La.
The next large branch from the main trachea runs largely across the dorsal surface of the muscles inside of the head, apparently supplying them.
A small branch runs from the main tracheal system to the supraesophageal ganglion.
Just in front of this ganglion, and lying so that the main tracheal branch is just behind the body of the tentorium, is the first transverse trachea described above. There are two branches from this trachea extending backward. Some- times they go to the subesophageal ganglion (Fig. 18), and sometimes they end in the nerve cords anterior to it. In sending these branches to the nervous system, this trachea resembles tracheze 2, 4, etc., but in lying above the neural commissures it differs from them. fn describing the main tracheze from the spiracle I said that two large branches extend toward the head. From one springs the half of the dorsal transverse system forming the diamond. From the other spring branches 2-a, I-a, the trachea to the supraesophageal ganglion, and the first transverse connection, I believe that these branches indi- cate that this trachea is the main longitudinal trunk, and not the one giving rise to the diamond. For, as will be shown, the transverse trachea? supplying the ganglia of the body segments posterior to the prothorax always spring directly from the main trunk near the spiracle. If it is true that the anterior arm of the tentorium is formed from an invagination of the body wall representing the spiracle



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19291 Tracheal System of Pyrausta nubilalis 341 of the mandibular segment (Comstock and Kochi lgOZ), it is not unreasonable to imagine that the first transverse tracheze indicate, at their point of origin, the positions of the branchings from a spiracle, and that it is here that the main longitudinal trachea ends, rather than at the pro- thoracic spiracle.
From just in front of the base of the transverse trachea two tracheze of about equal size run forward. The inner one ends in two short branches, one to the maxilla, and one to the labium. The outer trachea sends branches into the mandible, and a branch to the region of the base of the antenna.
I was unable to trace this into the antenna. A small branch from the trachea to the supraesophageal ganglion runs forward and joins one of the mandibular branches.
The other tracheze arising from the prothoracic spiracle are the posterior branch going into the formation of the main longitudinal trachea, and a branch also running back- ward which joins the trachea to the posterior apex of the diamond. Branches also go from this backward running branch to muscles and fat bodies, and a branch divides into tracheze supplying the appendages of the mesothorax. T~acheg to the Nervous System
The trache~ to the supra- and subcesophageal ganglia? and to the prothoracic ganglion have been described. The mesothoracic ganglion is supplied with air by tracheze 2 and 3. Trachea 2 sends a branch backward to the ganglion, and trachea 3 sends one forward as described. Trachea 3 lies on the inner surface of the muscle layer and sends many branches to the muscles (Fig. 15). Its main branch runs around the edge of the muscle layer and? turning downward, extends into the mesothoracic leg. Trachea 5 differs from trachea I and 3 in that it does not connect with the corresponding trachea from the op- posite side. It sends branches to the muscles and its for- ward branch runs around the edge of the muscle layer, and turning downward, enters the metathoracic leg. It also



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342 Psyche [December
sends small branches to the metathoracic and first abdom- inal ganglia. (Fig. 19).
The metathoracic ganglion receives, in addition to the small branches from trachea 5, branches from trachea 4. These run backward, one on each side, to the ganglion. (See Fig. 14, representing trachea 2).
The first abdominal ganglion receives, on each side, a small branch running forward from trachea 6, as well as branches from trachea 5.
(Figs. 16 and 17).
The abdominal ganglia receive their air supply from the transverse tracheae. In the abdomen these all are found on the outside surface of the muscle layer. The tracheae corresponding to tracheae 1, 3, and 5, do not enter the ner- vous system. Another difference is that in the thorax the transverse tracheae of even number, 2 and 4, lie in front of the ganglia and send their branches backward to them, while in the abdomen the transverse tracheaa lie behind the ganglia, and the branches to the ganglia run forward. The tracheae that form the transverse tracheae in the abdomen divide into an anterior and posterior branch. It is the posterior branch that meets the corresponding trachea from the other side, forming the transverse connection, and from this branch, also, that the branch divides that supplies the ganglion. The anterior branch may send branches to the ganglion, but apparently more often it does not. (Fig. 20). The seventh and eighth abdomnial ganglia have coalesced and lie in the seventh abdominal segment. The trachea that normally would supply the seventh ganglion here supplies both. There is no separate connection between the eighth ganglion and the tracheal system. (Fig. 21). Peterson (1912) figures two transverse tracheae in the tomato-worm larva sending branches to 'these ganglia, the one supplying the seventh passing under the eighth, and the trachea sending branches to the eighth is apparently the transverse trachea between the spiracles of the eighth