138 THE SPINY ELM CATERPILLAR 



never caused such widespread destruction as the Forest Tent 

 Caterpillar has recently caused in New England. In the 

 tree plantations of the prairie regions of the West, these cater- 

 pillars are, according to Prof. Otto Lugger, "very injurious 

 and sometimes denude whole rows of willows and poplars." 

 The same observer adds: "They are still more fond of the 

 leaves of elms, and I have seen them so numerous upon those 

 trees in Michigan that branches were broken by their weight." 

 In other regions of the West similar records have been made. 



NATURAL ENEMIES 



Like most other insects the Mourning Cloak Butterflies 

 have many natural enemies to contend against. From the 

 moment the egg is laid until the butterfly' dies it is iji constant 

 ■danger. ' T ,' 



Some of the eggs never hatch into caterpillars (because a 

 tiny fly, scarcely one twentieth of an inch in length, finds the 

 egg mass, and deposits in each egg another egg, — the latter 

 being microscopic in size. This tiny egg soon hatches into a 

 maggot almost equally tiny, and. the maggot grows by absorb- 

 ing the contents of the butterfly egg. In due time it changes 

 to a minute pupa, later to change to a minute fly like the one 

 that laid the microscopic egg. This minute fly now gnaws a 

 hole through the egg-shell of the butterfly and comes out into 

 the world. The little creature that has thus developed at 

 the expense of the egg of the butterfly is called an egg para- 

 site. There are many species of these egg parasites and they 

 attack the eggs of many kinds of insects. The particular spe- 

 cies that develops in the eggs of the Mourning Cloak Butter- 

 fly is called by scientists Telenomus graptce. 



After hatching from the egg the caterpillars are subject to 

 the attacks of various parasites. One of these is quite 'mi- 

 nute, — not a great deal larger than the egg parasites. It is a 

 tiny four-winged fly which deposits ma'ny eggs in a single 

 caterpillar. The eggs hatch into tiny maggots that grow at 

 the expense of the caterpillar, finally killing it and changing 

 to four-winged flies again. As many as 145 of these para- 



